Restoration is the best, in terms of number of useful spells. Alteration would be my favorite but with the removal of Levitate it's just middling. Destruction is good, but severely nerfed compared to its Morrowind glory days of town-decimating super fireballs (magic in general is nerfed compared to Morrowind). Illusion, with the removal of the Chameleon spell and the ease of producing potions, is totally worthless.
Sorry I almost completely disagree with your anolysis.
Restoration is the worst imo in terms of useful spells because its possible to go through the entire game casting only the beginner lvl healing spell.
As for wards:
*spoiler*
Spellbreaker is a deadric artifact shield that casts a permanent ward whenever you block with no charges and no cost of magicka.
Alteration has an amazing perk tree and spells with great benefits for those people who don't want to abuse the hell out of Enchanting, Alchemy, Smithing synergy and still want to play at high difficulties as a mage. Imo its practically invaluable.
Destruction is good, it would be great if higher lvl destro added to damage from spells and/or if it was possible to at least upgrade the lower lvl spells to allow for increased flexibility of spell usage. Spamming thunderbolt and chain lightening is kinda meh -___-
As for Illusion I also find this school invaluable because of CC (crowd control). Fear and Calm are amazing ways to control crowds and save your butt from deadly mobs of bandits at higher levels.