I guess my favorite combination would be pre-hardmode Corruption and hardmode Crimson.
I like the Corruption color scheme more. The trees look nicer, the demon altars and shadow orbs look cooler. I like to build a minecart track through the horizontal cavities in the early game, and you then drive by all those altars and orbs. Plus, I still like the EoW. Okay, it's very easy to beat, but it's a nice concept. I also prefer the Corruption surface music.
During hardmode, it's different. The Underground Corruption monsters annoy me for some reason. I prefer the Underground Crimson monsters, and I also like the music of that biome (unlike the surface music). Fortunately, that's not an insurmountable problem, as I just keep Underground Corruption in check and plant some Crimson in the Corruption world, so all's well.