I'd have to say The White-Luck Warrior. It just felt like the moment where the series was really shifting into second gear. Nonmen, Sranc, Bashrag, Dragons, Yatwer and all the stuff with the Gods, the Great Ordeal, the Judging Eye, Meppa...it just felt like more FANTASY to me. Like up until then all of these things have been sort of skirting around the edges of the main narrative, and now they're finally coming together. TUC, obviously, is going to be where it all really hits the fan, where all (or most) of the pieces of this gargantuan sandbox universe Bakker has created are in motion.
The Warrior-Prophet I suppose comes second, but TTT might beat it out. I feel like I enjoy TWP slightly less than most people (it tends to be the favorite), but it's also the biggest book. I honestly think I need to read it a third time, because there's SO much that I know is still vague for me, or that I'm missing out on.