I second that. I took a pure illusion (with some conjure) mage to level 100 skill, and it was a force. I'm guessing some of you saw the old Oblivion guide to beating the game on Very Hard. It boiled down to choosing the Wood Elf, because it was the only race that could realistically survive the first encounter with multiple rats in the sewer tutorial (using the racial power to turn one on the other), and then the rest of the game you had to focus on Conjure and Illusion. Skyrim seems to have continued the trend of making these two schools the most powerful.

In my opinion, I think Alteration is the best.

edit - not for the spells, but it has some really good perks available when you get good with it.