My favorite style is the one I have been putting into wograld (www.wograld.org) it is 45 degree isometric with a cartoon style sort of anime, but with larger characters. I like the bold bright colors, the sense of movement, and the fact that it is sprite art rather than being rendered.
My least favorite style is blocky 3d with lots of gray and brown and bad photo realistic textures. I've seen games try to splice things from a photograph to put cardboard cut outs in game, as well as moving polygons where you can not even tell what they are. For a long time I refused to play any 3d games cause they looked so awful.