I don't watch enough school anime that actually focuses on school, or all of the uniforms just sort of blend into one big blah mass of the same thing. But since I have to choose, I really like the uniforms from Medaka Box (excepts Medaka's). I like both the student council's and the regular students uniforms (although, they're technically the same). And I like Uotani's Yankee length uniform skirt from Fruit's Basket.

On a side note, prize for outfit that could be mistaken as a uniform even though I'm pretty sure your school doesn't have uniforms goes to: Yuno Gasai! I just kept assuming that it was a uniform, but nobody in the classroom wears anything even remotely uniform-ish besides Yuno...