They're both neck and neck for worst expansion to me. Both sucked in their own ways as have been pointed out (Wod - lack of content but okay classes/systems, BFA - tons of content, shit classes/systems).
However, choosing which turd was the worst of the two, I'd have to go with BFA being worse (at this point). WoD had fuck all to do yet I was subbed most of the xpac (I enjoyed leveling up multiple alts and just fucking around) whereas I'm already unsubbed to BFA (with my real life situation and time available all being relatively similar).
BFA can still turn things around but I just don't see it. I just don't enjoy anything about this expac at the core, fundamental level. Not the features, classes, or systems.