There is a whole heaping pile of shit I don't agree with in this thread, that's for sure.
Vanilla was bug ridden to no end, yet in many ways, a very fun game. AQ40 and BWL will forever be favorites. The PVP system was kind of lopsided.
BC was incredible. It's PVP system clicked for me, and I really enjoyed the raids and attunement systems. Twinking was amazing here for 59s and 60s.
WoTLK was all over the place. The PVP outside of twinking was a mess, in my opinion. I loved the raiding, despite the introduction to more casual raiding. Despite that, gems like Ulduar are impossible to overlook. I had the most fun twinking in WoTLK of any expansion.
Cataclysm was a heaping pile of garbage. Besides some of the quest changes and Sinestra, it was terrible.
Mists was pretty fun. I enjoyed Heart of Fear quite a bit. SoO was well done, yet the lack of content for that long was horrendous.
WoD was about on par with Cataclysm, but worse in the aspect of lack of content and release timings of content.
I personally believe the best model is trickling out content, and longer times between expansions. But yeah, that's opinion, really.