The fact MoP was the only expansion to stop WoW decline by around 8M active users. Not only that this expansion had more features combined then last 3-4 expansions.
Not only that they were top tier features. Timeless Isle is still the best end game isle they created to this day.
Not only that Isle of Thunder was proper made zone that even made time gating felt like fun. due the fact all the realm had to join in to unlock it.
200 man Oondasta.
New Class, New Race, Challenge Mode Dungeons, flex raiding, Farm, Pet Battles, Scenarios, Brawlers Guild, Proving Grounds, BMAH, World bosses return, Best Raids since ages since MoP. (ToT, SoO) and the list goes on.
Only people who says that MoP was shit haven't played the expansion at all, it was beloven till the end. (Hated at first then loved) only thing people complained on was the amount of dailies 5.0 had.
WOTLK Being most popular doesnt mean it was the best or the most favourited? Hell if you look at graphs the decline started mid WOTLK and only reason it bounced back to 12M was because Cataclysm was announced. But hey WOTLK best expansion right?