Erandur or Illia>Marcurio.
Illia isn't even a mage, but a Witch.
No, and here is why:
Illia - She is a Mage that focuses heavily upon Frost related spells.
Erandur - He uses destruction magic with one hand while welding a mace in the other, but at times he'll even use a bow.
Marcurio -
Dual wield destruction magic, duel wield lighting bolt( killing 2 enemy's at once in 2-3 casts) , he has a spell to make undead,vampires,draugh flee, he casts fire spells and has frost shield
Sorry but marcurio is better, even if they cast same spells he still uses duel wield magic while the other 2 don't, plus the lighting magic has a huge range and its good vs groups
Oh and the person who is writing the skyrim wiki said once that marcurio is his personal choice.
And if i am not mistake one of those 2 can summon familiars wich is kinda too much in a dungeon than can block your way and you have to reload the game.