Not for the most powerful summon, rather for the most fun summon. Which is it? If it's another conjuration, which is it?

Thrall can be fun, don't forget to reverse pickpocket them with some sweet gear before u raise tho. I still prefer my dremoras
"I SMELL WEAKNESS" Twin summon then mix yourself a drink, let the gladiators do your dirty work for you.

dead thrall a dremora lord i haven't tried it but I heard it's possible most fun is well it depends you can use dead thrall on certain npcs (Cicero) and dremora lords have really nice dialogue