BfA by a wide margin.
WoD was much better despite not having much end-game content to do, but that expansion had a way superior class design/balance, PvP was hands down much better, the raids were very good, I did a lot of things with my alts without having to grind mindlessly like a bot 24/7, the new zones were very good (I just loved Draenor) and I simply had much fun playing WoW, something I have barely had during a few months playing BfA (8.1, Dazar'Alor raid tier).