I have to vote Illusion because Charm and Paralyze make my life so much easier. Destruction is a very close second, though. I only ever draw my weapon anymore if I feel like a challenge (and I have Blade at 100!).
Yeah, that's a pretty solid strategy, except it seems to me like Destruction takes FOREVER to level up (I've maxed Illusion, Conjuration and Alteration, and I've nearly maxed Restoration, but am still less than 50 with Destruction...)
I managed to get Destruction to 100 even before Illusion and Mysticism, which I had as Major Skills. Here's how:
1. At the spellmaking altar, make Destruction spells that are much more effective if you cast them multiple times in a row, like a combo of Fire Damage and Weakness to Magic. Cast back-to-back, you can double or triple damage on subsequent castings.
2. Again, at the spellmaking altar, mix Destruction effects with other effects so that spells you use routinely end up leveling Destruction for you. For instance, mix in some Fire Damage with Invisibility to set enemies on fire and make them forget who they were fighting at all. Mix in Frost or Shock damage with a second of Paralyze to keep enemies down as you repeatedly decimate them. Mix in 100 points of Drain Life with a 1-second Soul Trap and cast that instead of using weapons with Soul Trap effects to finish off enemies.
You can't beat Illusion for convenience, but I also get the impression that you can't beat Destruction for quickly eliminating a single or small groups of enemies.