Definitely conjuration if we're considering best to = most powerful. Dremora lords or storm atronachs can handle most of the combat in the game on their own. The bound weapons are pretty solid before you do the smithing grind as well, early game they can be a major advantage and the bow is the best to use for leveling archery skill. It's not a very diverse school but it makes combat very easy.
Next would be illusion for pacify which gives you complete control over everything but a few immune enemies, and of course quiet casting perk and invisibility for stealth.
Restoration isn't amazingly powerful but it's definitely good to have up to maybe adept with regeneration and respite for convenience. Wards are unfortunately garbage due to bad implementation of the concept along with just unreasonably magicka efficiency, and turn undead while useful against a common enough enemy type, isn't worth spending extra perks on when you can just get the perk for illusion to affect undead eventually.
Alteration has good perks for the passive resist/absorb, but the armor spells are worthless and it's mainly useful for convenience - detect/candlelight/transmute are nice to have. Paralyze would be good except it's competing with pacify which overlaps it in function to some degree, and i'd say it loses.
Destruction is a complete dud unless you're going 100% cost reduction along with potions to boost it. They had some interesting variety for once but they just made it so inefficient and poorly scaled that all the new forms like runes/walls/cloaks and so on were kind of for naught.