Let's be honest, Destruction magic has been severely nerfed. In Oblivion, creating custom spells like giant 100 ft area fire balls just made destruction magic wreck everything.
Now...destruction magic is probably the highest dmg you get at the beginning of game but once you improve enchanting and smithing swords and bows just become epic weapons while destruction has no variety, no custom spells, and doesn't improve in dmg much as you progress.
Restoration is useful for obvious reasons but obviously only as a secondary use and once your character is decked in armored up and enchanted gear you don't have to wort about restoring health again except against the hardest opponents on the hardest difficulty.
I guess enchanting is technically a magic class which would make it the most useful, indeed, by far but I'm only counting spellcraft magic here.
Finally conjuration magic is no doubt much stronger in the late game of Skyrim compared to Oblivion. Oblivion had more variety in conjuration but it was always only for secondary help once you lvled up and the broke lvling system only made even opponents like typical guards become super annoying if you lvled too much.
Skyrim though, I mean two dremora lords is no variety as that is pretty much the only uber set up for conjuration you can obtain besides lucky corpses to resurrect with the master spell. But eventually they will die and you won't be able to find their bodies again or just disappear in a glitch as I've experienced before. Dremora lords are thus the ONLY really strong summons with Storm Atronachs still svcking by comparison and only more useful for dragons. But though there's hardly any point to variety in conjuration anymore, there's no question whipping out your two dremora lords at the start of any fight almost ensures victory.
But now Illusion...you know in oblivion I don't remember illusion being that useful at all beyond invisibility and for a while I thought the same in skyrim. But one thing changes that: DUEL WIELD PERK...
Most enemies in skyrim no matter what lvl you are (even if you have the legendary edition like me and go as high as 300) do not exceed lvl50ish. But that means both useful and entertaining spells like frenzy can easily affect even the strongest opponents when you duel wield.
Playing as a weakling character now who has hardly any armor and hasn't improved archery yet as that's her only real weapon, illusion magic after focusing in that instead has proven to wreck everything. It makes sense that there are a few opponents in skyrim that aren't affected at all by illusion spells (except invisibility which ALWAYS works) because truthfully you can destroy almost everything else without wvwn lifting a finger.
Illusion is also the only magic class where the master spell is actually...MASTER..Mayhem
especially for its huge range, passes through walls, and affects up to lvl37 which is actually most opponents still. For draugr, the only ones not affected up to lvl37 are the strongest death lord archers and a overlord. It affects every bandit type and most vampires besides master ones. Probably the only type of opponent mayhem won't help much against once your lvl 50 and higher are falmer since most will be shadow masters at that point who are too high.
But even all those opponents still get affected by duel casting frenzy and the like. And in any event, Mayhem usually still affect most enemies in most situations.
In fact, after completing the first Companions quest and reaching the end where you most battle a draugr army before you can go, I forgot about it and became very nervous. I was lvl 52 so I knew most would be death lords yet I had barely any armor or health still. Then I remembered "Mayhem baby!" I chucked the spell out and immediately every single draugr broke out of its crypt and the army just started attacking themselves while I hid quietly in the corner sniping some with my crossbow until they were finally all dead.
So Illusion...I think most players still assume it's not that useful in skyrim but in reality it's probably the strongest weapon in the game if you use it right: invest in ALL the perks, use illusion cost less enchantments on gear, and invest in sneak too so invisibility becomes uber as well besides frenzy and Maybem. Even calm is incredibly useful in fighting when you just calm an opponent after sneak attacking them then sneak attack them again lol...rinse and repeat...calm was actually the only way I could beat some opponents playing on master difficulty and on the first levels where fury is not that strong to be useful yet.