I think it has to be BC or WoD. WoD might get a partial pass for taking place in an alternate reality where things aren't QUITE identical to the main reality's history. It's still a worse overall expansion due to lack of content/quality content though.
Both BC and WoD suffer from story problems. The Illidari faction was always somewhat evil and ruthless, but BC offers poor reason for us wanting to kill them off aside from the need for raid bosses. Illidan himself was completely wasted, Blizz realized this pretty quickly and tried bringing him back and explaining him better in Legion, semi-successfully IMO.
WoD was just half-assed in all respects except visually. We did get some great Arakockra lore even if the 'threat' of the Iron Horde.
In BFA, while the main Faction War story is garbage, but individual zone lore is strong... well, Drustvar is strong, but the other five zones are still good.