I'd go with Marble, because it's rather empty and boring. Every area is the same: A marble ceiling, a marble floor and two cut-off edges to whatever biome it generated in.
(Would be nice if the devs would flesh out this area some more.)
I don't consider marble, granite and spider caves a biome on themselves, they're just a part of the biomes they generated in.
So, my vote goes to Crimson. I still like it, there's not much I dislike in this game, but yeah, because I have to choose one: The fleshy meat theme is rather gross. Not to mention the Crimson furniture. Yuck.