The elder scrolls wiki has a great page on the 'best' thralls...you should check that out.
Honestly though targeting specific npc's is the worst use of Dead Thrall in my opinion. They get lost pretty easy and can spoil gameplay because you're forced to micro-manage them (Character Build: The Childminder)...
...it's far better to thrall up some dudes while playing naturally and maybe take them for the occasional dragon fight. Marauders, good casters...npc's that can be easily replaced if/when they get lost.
I do not remeber where I've read this, but some unique NPCs (I remeber Ulfric was mentioned) keep getting XP as thralls. So if you use them and they die, it is possible that you won't be able to reanimate them because they exceed the level cap (40, 50 if necromage is unlocked). But anyway I plan to create a character based on dead thralls, Invest a lot in crafting skills to improve the thralls' gear :D
Forsworn are great thralls, you can easily grab more if you lose them as forsworn camps reset every 10 days. The high level dual wielding ones have nearly all the One-Handed perks making them very efficient killers. The archers have Power Shot and Quick Shot, and they can heal themselves. The "Forsworn Ravager" found at Karthspire Camp can summon a Frost Atronach and use the adept level spells. The best part is that you can easily replace all of these thralls if you lose them.