//edit: JaronK, you said Animals, Dragon, Magical Beasts and Giants make the best minions. That leads to the question what methods are best to get those. I don't know any other method right now than just rolling knowledge where the most of one kind live, go there and kill them, but I'm sure there are some spells that could help (summon doesn't work because the summons disappear).
As a Dread Necromancer, even Dragon Ally is off the table... so basically, go find them. Luckily, it's pretty likely you'll face at least some of that group during your career (they're pretty stock standard enemies).
If you find Humanoids, the Animate Dread Warrior spell is best for animating them, though that obviously requires either Advanced Learning or Spellstitching.