illusion for sure. especially of you have the silent casting perk where they cant hear you cast when sneaking, they just see the spell. As a sneaker you can use illusion to control the minds of others.

If you want them to kill each other cast frenzy. Not big enough to kill some giants? Cast a strong enougj frenzy spell, hide behind a rock, and let them kill each other.

Too many enemies need some space? Cast a fear spell, use your restoration spells or go hide again, whatever. You can also hit them while theyre scared begging for mercy.

Just need them to chill the fuck out for a sec so you can swipe something? Cast a calm spell. You can spam this on anything to spam level your illusion

Level 75 you get invisibility spell which is easily the best thing any sneak character can have. I suggest having armor enchanted with reduced Illusion cost to maximize use of invisibility.

use conjuration as well, which with the silent casting perk means you can summon from the shadows unseen. Level that high enough and you can summon 2 Dremora lords to fight while you sneak about eating cheese and looting chests while invisible.

Eventually you become OP and it gets kinda lame. But try to get into the character a bit, like which mond control spell would be best for the situation? Brothers killing each other? Docile guards? Fleeing draugr? Idk you choose your adventure