I went for Uldum. WotOG was surely also great, but I think Uldum had a more diverse meta, that still enabled many aggressive and defensive deck strategies. I don't recall any deck or class being particularly problematic either. Unlike Ungoro, that had rogue and freeze mage driving people mad, KotFT with Priests and druids en masse, Gadgetzan.... XD .
Uldum seemed exceptionally unproblematic. It made the meta slower in contrast to RoS and introduced my favorite card (corrupt the waters).
The only thing I really disliked about that expansion was puzzle box, which wasn't THAT bad for the game in the end.