Glazed terracotta, without doubt. It's the reason I'm got back into playing, and the reason I stopped building underground and moved on up in the world. If I were to pick a 'best' of the colors, it would have to be between White and Orange glazed terracotta; they both have great color interaction and can bridge a gap between warm and cool colors.
(Off topic: Heck yeah AtollaBell, represent for the vibrant blocks!)