Elderscrolls Loreheads generally agree Destruction takes top spot, followed by Illusion, purely based on in-game NPC class lists and leveled spawn data. Playing on max difficulty, you just get swarmed by fireballs and frost atronachs from every bandit mage and conjurer dungeon delve. Restoration's high up there too – all those vigilant types and temple healers wandering Cyrodiil.
But Oblivion's scaling system kinda flattens specialization distinctions compared to Morrowind's rigid guild ranks. You'll find 'apprentice' labeled mages slinging expert-level spells because their level tag bumped their magic skills. So the 'most common' schools reflect generic enemy templates more than deep lore choices.
Still, from a pure gameplay hours standpoint, Destruction dominance checks out. Even the Mages Guild recommendation quests force you through elemental trial caves packed with flame mages.