Hey man, only just joined this Forum, but I was coming back and forth to this thread with constant interest so I'd thought I'd post my story.
So yeah, I went collecting the strongest NPC's I could find around skyrim after some inspiration, journey started at about lvl 26, ive got about 14 corpses all lying around in Drelas's Cottage and none have vanished so far. (Don't have any Skyrim DLC's, might get Legendary edition at some point)
Drelas - Awesome Elemental Mage, whatever picks your fancy, Ice, Fire, Storm, best to catch him at later levels when he has learned his Elemental Cloak Spell.
Haldyn - (East End Company Quest) - Strangely, Even though I fully ended the questline - for me, he's still battle active, his AI still works
Orchendor -(Peryite Deadric Quest) Powerful despite his Frost dualcast preferrance, was hoping he'd scrap that behaviour if I met him later. No Luck.
Sulla Trebiatus -(Aftand) - Need to test him some more
Umana - (Aftand) - Need to test her some more
Galmar Stone Fist - Stole him when I ended Civil war quest, might aswell.
Ulfric Stormcloak (my whole build was made for this guy as my thrall, and its WELL worth it, HE DISARMED ME!)
Malkoran - (Meridia's Beacon) Dualcasting Ice storms pretty fast he's a good thrall.
Malyn Varen - (That Azura Quest) - Pulled him out with a Dremora Malkynaz BEAST, but the Dremora wasnt permanent corpse - vanished after a few days, and it got a bit hectic with his constant fireball barrage and my other allies werent getting any action, if they did, usually it was hacking away AT HIM!
Malyn was pretty weak when i tested him, no cloak or conjurations sadly.
Bashnag - (Southfringe Sanctuary) - When I encountered him, he was great, Casts Frost Atronach in battle frequently and uses Ice Spike, also came with 5 pieces of enchantable equipment, dont think he knows Frost Cloak or Ice Storm, because if he did use them I would have him as a permanent thrall beside me. I encountered him at his last thrallable level without the Necromage perk, so theres still more levels I missed for this guy, but i dont plan on becoming a vampire.
Chief Ymarz - (Cursed Tribe Quest) TANK. That is enough.
Madanach - (Forsworn Conspiracy) - WAS Cool, thralled him at lvl 33 and he casts ice spike very quick like bashnag and enters battle with a Frost Cloak, doesnt conjure but is aggressive with melee, his battle AI Screwed up when I left him in Drelas's Cottage doing some missions, came back and he stopped attacking me when I attacked him (I Do alot of AI battle testing with each of my Thralls to test they can ACT in battle and wont just stand still) if they attack you, they will act normally as your Thrall and will face any foe accordingly.
Still need to do more testing on Him but I think he's perma screwed, I killed the other forsworn Named people at the Druadach Redoubt (where they all run to at the quest end) and Borkul The Beast actually works as a thrall after end of quest, so my guess is, if any of them are thralled before reaching Druadach Redoubt or before the end of the quest (To finish the quest you have to watch the forsworn escape markarth and kill the guards)
their AI will stop working at some point later, could be a cool test for someone, I reckon Madanach would work using this method.
Anska (Anska's Scroll) - Pretty decent. Have no knowledge as to whether she can reanimate or conjure, not seen her do either yet.
Estormo - (Thalmor Sneak in Quest) - Re-Thralled this guy later to find he uses Flame cloak, Thralled him cause he was the first person I encountered that Conjured an atronach, casts frequent Lightning spells for me, so he's using two elements.
My two CURRENT Thralls (not stored in Drelas's Cottage, and my Favourites) are:
1. A Forsworn Pillager, who despite moaning, Casts Ice Storms more frequently than Spikes, Uses Frost cloak on battle entry AND conjures a frost atronach, tested her vs Ice Forsworn Ravagers and she Owned them simply because they use Ice Spike (Both conjured Frost Atro's so them two were just smashing eachother while the women fought Womano - Womano.
2. Movarth Piquine - I believe I'm the only person who's got his Battle AI working (ALOT of testing, loading, attacking him) after the Blood on the Ice Questline and at present he is clad in Orcish Armour (torso, Boots) Kicks ass and has an undead battle cry that goes "rrrruuuuuuueeeeaaaaagghhh!!!" but sounds awesome.I re-rolled the dungeon until he had the correct radiant face that I wanted (some movarth piquine versions looked really pathetic) and then, I just looted everything from him, re-thralled and repeated until he had armour. He isnt permanent and dissapears after a few days, and doesnt respawn in the dungeon even if the other vamps have, so my only choice is to keep him as a permanent thrall or I lose an awesome vampire.
Stuck at lvl 33 I plan to level past this soon and perma thrall Ulfric, at the moment though Im going round getting strong npc's, because soon ill reach the Thrall cap, and some thralls, even past 40 are thrallable if you meet them earlier (the only ones I wasnt able to thrall at lvl 40 in Drelas Cottage, were Ulfric, Estormo. The rest I could still reanimate.
Enchanted fortify health by 65000 on ulfrics torso, didnt work till I levelled up and rethralled him, dudes unkillable now and has an absorb 100 health sword and block 30% more damage Deadric Shield, tried killing him on all difficulties, and I couldnt with Beserk Rage, White Vial 50% dmg bonus, Dual wielding, Elemental Shout. So Awesome. He shouted at me "At least TRY to hurt me!" Made my day.
Dead thrall is like, one of the main reasons why I still play skyrim, every now and then I'll come across someone that just has to be my underling!