I wouldn't say there is a 'best' or 'worst,' it just depends on what is most beneficial for your archetype.
But, for the sake of ranking, I would pick Conjuration. The Bound Bow is extremely powerful, and once you get up there summoning a Dremora Lord is just so freaking awesome.
Restoration, because any build can make use of it. You've never been out of potions and dual wielded healing, retreating and healing with your level 17 restoration skill?
They're all really good. Destruction is still good. Have some fortify destruction potions of your own on hand and one of the dragon priest masks from Dragonborn and you're pretty set to lay out the hurt. However, I always urge against going with just one school of magic as they seem to all work well with one another.
Haha, only every other fight on my no-armor pure mage.
Though by now it's much higher than 17, even though I have no desire to skill it up.
I can see how conjuration must feel pretty wicked to folks who already have it up to master level, but prior to that, flamey and frosty are underwhelming. Frosty can take a beating and has that cool stun move and flamey does pretty good dmg, particularly vs melee enemies, but their bad AI combined with their natural weaknesses can make them pretty frustrating. Looking forward to me dremoras to see what all the fuss is about.
Likewise with Illusion, being able to send a room full of enemies into a tizzy, attacking one another, is probably pretty awesome, but at my level it seems like all I can do to help my atronachs in battle is to Calm one of the enemies attacking them. And even then I can't do diddly against undead.
Destruction is the only school that feels right at lower levels. I suppose it's no small irony that destro's effectiveness wanes at endgame whilst the above two become OP at endgame.
IMO I think Conjuration and Illusion are the most powerful. All of my evil characters really benefit from Conjuration, between the bound weapons, raising the dead and summoning Dremoras is pretty sick. Being able to control the minds of your enemies IMO with Illusion really doesn't get any more powerful than that.
I always prefer to play mages and have found conjuration to be the most useful as I don't carry any weapons. An antronach or two is extremely useful in tight situations, especially the Dremora twins later in the game.
Illusion is definately a patience skill. You need a level of 100 and all the perks for it to be most effective. Its basically all or nothing. But once you get it, with the master of the mind perk, you can even have draugr deathlords turn on each other.
Yeah, outside of 100 and no access to pacify, Illusion is slow going.
Depends what the context is. For pure stats and numbers, Enchanting is the best magic school if you get the double enchanting perk and a few others. But if you don't want to count that because it's a crafting skill, then Illusion is the next best for pure stats and numbers. Mayhem, with the perks to make it better (and if you're a vampire to make em stronger), and the perks to make it so they work on undead and dwemer constructs, you're basically unstoppable.
For overall usefulness for any class, restoration is pretty useful cus even people that don't use it can still use it as a last resort to heal themselves a decent amount.
Conjuration is pretty powerful too when you get it high up there and get the better summoning spells (storm atronach, Dremora, Thralls, Wrathman, Ash spawn, Ash guardians), and when you can revive some of the more powerful draugr to fight for you is always nice too.
Conjuration. Love me some Dremora smack talk!
Really, Conjuration and Illusion are the best schools of Magic in Skyrim, alteration is also useful.
Conjure Dremora Lords is the best spell in the game.
All of them. Magic is the most fun part of the game.
Maybe not what the OP meant but I pick Enchanting. Best school for every type. For a Mage, obviously, you get 100% cost reduction in two other schools. For a warrior, you get a 172% damage bonus plus and extra 156 magic damage on your weapons.
My favorite school of magic is Conjuration. Ties in to enchanting well, can match what destuction spells I'm using with the summon I have out, provide great tanks for when I want to pull off a master level spell, allows me to use melee combat when the mood takes me. Lastly I find a summoned creature to be more reliable then raised ones and mind controlled enemies.
Never used Conjuration much, but Illusion has just made my Khajiit a total boss. He's so powerful, he hardly ever kills people or animals, he just hits them with Rout and they run off. Drager, on the other hand, he terminates with extreme prejudice. Dead people should stay dead.
http://i.imgur.com/02PJXlr.jpg
http://i.imgur.com/q8ZM0Tz.jpg
Personally I think the best school of magic is Illusion. That being said, you have to really be willing to go all in to the Illusion magic skill for it to be really effective
Best school of magic in Skyrim? Let's see: