Crimson all the way for me. The skull in the background, boss, and spreading gore make for the compelling possibility that it could be the regenerating body of Cthulu, and that is far more interesting to me than the more general darkness of the Corruption. The variety of enemies, the color, the screams of the face monsters, the music, the actual BoC fight, Ichor, just love everything about the Crimson. Plus, I vastly prefer the terrain shape over the cavernous drops in the Corruption.