You want the Best you can getEdit
A second level Cleric could theoretically use Rebuking to command a Paragon Ghostly Visage. It would have a Save DC on its paralyzing gaze of 39 and completely own every monster you ever encounter for many levels to come. It doesn't even get a save, you just make a Turning Check and if you get a result that is capable of effecting creatures with at least 1 hit die less than your effective turning level, it's yours! Assuming of course, that you ever ran into one.
The problem is that this is basically Pun-Pun. If the DM happens to arbitrarily decide to give you power that is completely out of scale with your level, you'll have power that is completely out of scale with your level. So while there are all kinds of crazy things that you can control with Rebuking or create with Animate Dead, the fact is that in an actual game these killer combos are simply not likely to occur. The DM could have you find the Sword of Orcus and the DM could have you find the corpse of a Pseudonatural Great Wyrm Silver Dragon, but unless you're 14 the DM is probably not going to do that.
Necromancy therefore, is an ability with very little pre-game min/max capability. Whether you are controlling undead or creating them yourself, you are throwing yourself at the DM's mercy. Like how every fighter I've ever seen has eventually taken to using some magic weapon that was found as a trophy, only more so because D&D does not currently support an "available corpses by level" guideline. ***(I'm unsure how to formally suggest this, but if you wanted to figure out average corpse per character level, simply look for a creature with a high amount of Hit Dice whose Challenge Rating is equal to or lower than that appropriate for your party. My cleric is a necromancer of Wee Jas and based on an Religious and Natural knowledge check, learned that he could raise storm giants as skeletons with animate dead and they were likely to be the most powerful/easiest to find combo. Then, with a little Commune conversation with Wee Jas herself (as the spell Commune) I was able to discern the location of some of those chaotic beings in order to turn them into tools for promoting the law.)
However, here are some things to shoot for:
Level 2: A Cleric or Dread Necromancer gets access to Rebuke Undead, allowing her to control 1 HD undead. Ideally you'd want a Ghostly Visage, but in reality you're going to make do with humanoid skeletons. Likely you're going to be going up against generic Human Warrior skeletons because those are very little work to put together as a horde monster. They can make quite effective archers at this level and you can control two at a time. You can't replace them, so keep them in the back.
Level 4: The 2 HD undead just aren't impressive. At level 4, zombies are only a modest irritant, and any you command with rebuking are best use to shamble ahead of the party setting off traps.
Level 6: Shadows only have 3 HD. That's the big deal at 6th level, because while every 6th level party is going to have magic weapons all around (enabling them to go through Shadows like butter), the fact of the matter is that most CR6 monsters don't. Incorporeal creatures can't be hurt except by magic attacks, and having attacks that "count as magic for the purposes of penetrating DR" doesn't count. Of the 26 monsters in the Monster Manual that are rated at CR 6, only 8 of them are capable of hurting a Shadow. Against all others, a single Shadow automatically wins.
Level 8: Two common monsters come on-line at level 8: the Wight and the Ghoul. If you are given the choice, use the Wight as they are better in all ways. Wights are pretty hard core, and have control over their spawn. If you arranged things right, you could have a small army of Wights under your control via delegation of authority. Wights are really common and can be created by killing anything with negative levels.
Level 10: Theoretically, you could turn a Deathlock into your Pokemon at this level, but chances are your DM has never heard of a Deathlock, so you're not liable to meet one. Ever. The Deathlock has detect magic at will and is thus a great utility monster - but it's a CR 3 and you simply aren't going to be using one for combat at all.
Higher Levels: Rebuking does not keep up with the CRs of the monsters you'll be facing by itself. If you want to make a name for yourself with Rebuking at higher levels, you're going to have to pump it up. A lot. To pick up a CR 11 Devourer you'll need to crank your Rebuking up to level 24. Which is doable actually. More likely you're going to end up cranking your Rebuking Level out to 20 and still settling for grabbing a couple of monsters that show up in hordes for your CR.
Command Undead is a weird effect. It gives no saving throw when used on unintelligent undead monsters and has a duration that lasts for days. When you get it you'll be able to automatically seize control of the first CR 4 Zombie Minotaur that shows up. Zombies are a total waste for creating with Animate Dead because they have a low effectiveness ratio to hit dice. But Command Undead gives you indefinite control over them as long as they have no Int score, so if you happen to encounter Zombies they'll make a great HP sponge.
You can get things that are way out of your league. There are unintelligent Epic Undead out there, and with enough invested in Greater Spell Focus you could theoretically capture one even at low level. Then you could march them around slaughtering literally everything that screws with you at even close to your level. Practically speaking however, you're just going to grab any unintelligent undead that comes your way (in fact, if you encounter mindless undead and don't have slots left for Command Undead it behooves you to run away and come nack later once you've prepared new spells). Zombies of your level have about 20 hit points per level - which makes them decent enough sponges that they are worth healing between combats.
The golden spell is animate dead. Spells like Ghoul Gauntlet are pretty much crap (it gives you ordinary Ghouls, which are pretty underwhelming, but more importantly casting it at all reduces the number of Undead you can control - avoid this spell like the plague). Create Undead can be gained at various low levels by various means - none of which are good. Create Undead doesn't do anything useful until you get to a Caster Level of 15, at which point it can give you a half-way decent Mummy (and Mummy's can be argued to retain their class levels, making this a potent way to bring characters back from the dead if you don't mind the fact that they can't gain levels anymore). Create Greater Undead gives out Shadows as the first thing, so it's all about the world conquering army.
Level 5: Assuming that you have the Deathbound Domain, your Cleric can make up to 30 HD at one time. Now, you lose everything you already controlled if you make more than 20 HD of Undead, but you specifically have control over everything you animate in one go even if it's over the cap. So you could animate the skeletons of two Fire Giants at once. Once you outfit them in some very reasonably priced armor and give them access to some Large Greatswords, these suckers will dish out more hurt than anyone else in the party (2d8 + 18 damage is no joke). If you have Corpsecrafter, both Giant Skeletons will have 157 hit points, which makes them individually competitive with the entire party.
But where are you going to get Fire Giant Skeletons? Certainly not from beating them in combat, they're CR 10. You're going to try to convince the DM to let you go graverobbing in the Giant Town or something. And this is pretty much your life with Animate Dead from now on. Even at the lowest level you get it, you'll be able to very plausibly craft some bruisers that are going to overshadow the party Barbarian in tanking and damage outlay, but access to corpses is probably going to be very tightly controlled.
Level 11: You have Create Undead. And you know what? You don't care. You have Ghoul Gauntlet as well, and you just don't care. You can't do jack with Ghouls, and a caster level bonus just gets you Ghasts (which also don't matter).
Level 15: You can make Mummies with Create Undead. This is important, because Mummies are hard core. Also, the rules for Mummy Lords are extremely vague, but could be read to allow you to use this spell to bring your friends back to life. You also get Shadows with Create Greater Undead. That's key, because 3 out 4 of the CR 15 monsters in the Fiend Folio are still completely powerless against a single Shadow.
Level 18: You can now make much larger incorporeal undead, which doesn't really matter because while Incorporeality is an automatic win against many enemies, even a Spectre can't survive in the environment of CR 18 monsters that can hurt it. More importantly, you can make a Mohrg. Mohrgs are pathetic losers who are so weak that you don't even get XP for killing them. However, they turn any monsters they kill into Zombies under their control with no hit die limit. This means that with patience you can use your Mohrg at home as a complex Deathknell effect that gives you powerful zombies. Your house Mohrg is a coup de grace machine that makes Zombies for you.
The Importance of being DesecratedEdit
Desecrate is an effect that is of astounding importance to a Necromancer. Any Undead created within a desecrated area gains an unnamed bonus to all its Hit Dice, and the cost of using it is minimal compared to actually making undead in the first place, so failing to desecrate before making undead is inexcusable. Desecrate can also be used to cut off an area from sacred power (whether it is sacred to a good or evil god even, so the fact that Good clerics can’t normally cast this spell is one of the many reasons that in official D&D: Evil Wins), and even makes turning checks more difficult. That includes Rebuking checks, so beware that your Desecrate aura is going to interfere in you controlling your own undead, so plan accordingly. Desecrate is not available as a Wizard spell, but its effects can be replicated by Black Water (from It’s Raining Outside) or Lesser Planar Binding (as always, mid-level fiends come to the rescue of the arcane necromancer – A Zovyut can desecrate all day for free if you happen to be an Infernal Bargainer, and a Maurezhi can just do it).
Unusual Undead CreationEdit
The standard methods of creation are all well and good for the average character, but what if you insist on thinking outside the box? That’s a possibility. Here are some undead creation methods you may not have thought of:
Spawning isn’t just for controlling one Shadow with your Rebuking and then making a chain of spawn that will conquer the world beneath an ephemeral boot. Oh, it does do that, but did you know that the control the spawning creature gains over its progeny is an instantaneous effect that triggers at the time the power is used? That means that you can use shapechange to pick up the Spawn ability of your favorite high-end undead monster, and make as many minions as you want that will serve you forever, even after your spell has worn off and you go back to being a halfling in a bathrobe.
Create Undead WarriorEdit
In Unapproachable East there’s a spell called create undead warrior that has no cap on how many undead it can create, and which gives you full control over what it makes. The undead warriors are kind of disappointing, having no Con and a penalty to Int and Cha, the potential of lost feats, and only a modest Strength bonus to make up for the pantsing (btw, what you really want to make is Drow Rogue Undead Warriors, because that takes best use of their few advantages). Also it costs a ridiculously large amount of XP (though no money) to activate the spell. So on the face of it, you’d never do that right? Well, you’re not going to pay that XP. Rather than playing Thought Bottle cheese, you’re going to spellstitch this spell, because it’s a 6th level Arcane Necromancy spell. Then you don’t pay the XP cost, and you can make one Undead Warrior every day for the rest of your life to join your army for free with no limit to your control pool. This would initially be so cheesy that we wouldn’t even suggest it, except that it’s in the flavor text of the spell that the leader of the Thayan Necromancer, Szass Tam himself, is already doing just that. Weird, huh?
Necrocarnum allows you to make hats that bind to your soul and allow you to make a Necrocarnum Zombie that is really quite good. You can only have one at a time, and creating one does a pile of damage to you that can’t be healed as long as the Zombie is active. That sounds like it would be problematic, but actually it isn’t because you just cast False Life before making a Necrocarnum Zombie and take the damage to (temporary) hit points that you couldn’t heal anyway. Problem solved. Like all things Incarnum, if you’re willing to spend a very long time reading the book and then an equally long time explaining to your DM how it works, you can have a very effective power set with just a small level dip into an Incarnum class.
Interestingly, there is an entire Necrocarnate PrC that supposedly advances your zombie making. It doesn’t. The ability to make Necrocarnum Zombies for less lost HP is meaningless because as previously noted, noone actually spends HP for Incarnum Zombies. If you want a Necrocarnum Zombie, take a level dip and don’t look back.
Some Surprisingly Good UndeadEdit
Not all undead are created equal, and when you apply those templates, creatures will arbitrarily gain and lose all kinds of stuff. For example, a Remorhaz is ungodly vicious for their hit die. The heat is an Ex ability that improves their basic attack, so they keep it while a skeleton somehow. It’s nasty. Also, don't forget that any form of Giant Kitty is horribly powerful because pouncing in D&D is so very very effective.
Generally, it’s good to keep in mind what kinds of creature will pull through with the best advantages. Here are a few:
Hydras make good Zombies.Edit
A zombie loses the ability to make a full attack because it can only make a standard attack or a 3rd edition style partial charge each round. That’s fine for a Hydra, because they arbitrarily have the ability to attack with every head as a standard attack anyway! They don’t lose anything by becoming a zombie, and when you Awaken them, they get back Fast Healing, which is good times.
Outsiders make good ZombiesEdit
Zombies gain natural armor on top of their existing natural armor, so Outsiders made into Zombies go from very hard to hit to crazy-go-nuts hard to hit. Start with a Green Slaad, say, and its AC jacks up to 26. Start with a Marilith and its AC goes up to 32! Planetars only have 14 Hit Dice... you know where this is going, right?
This fact neatly puts Zombies into the "damage soaker" category of monster. If you can find T-rex style monsters with single large attacks and great AC, these Zombies can be quite competitive with their Skeleton brothers.
If you’re going to Awaken something, start with a Hellwasp Swarm Skeleton! Oddly enough, you can make a swarm into a skeleton. Once you use Awaken Undead, it gets its EX abilities back, and those are alarming. Fun tactics include...wait for it.....MAKING ZOMBIES! Not only are you bypassing the whole "caster level limit" business by making crazy HD Zombies with each Hellwasp Swarm, but you get to Dominate Monster as well.
You might even want to Revive Undead your Hellwasp Swarm every time it dies. Losing HD is actually good, since you then can eventually animate and control more Hellwasp Swarms.
Skeletons mostly keep their Movement Forms, and Zombies keep all of them.Edit
That means that a Bulette can still burrow, for example, just as a Scrag can still swim. That’s quite a bit of mobility you can pick up. Run around with a Thoqua Skeleton and you have a tunnel-maker on a stick, as their burrow specifically makes tunnels.
Skeletons don't get flying unless your DM thinks that they fly "magically". What this exactly means is beyond me, but it generally means that you can have a flying beholder skeleton but not a flying griffin skeleton. Zombies are all good if you reach a level high enough to cast Animate Dead and all you want is a cheapo flying mount.