All the expansions have had their good moments, and MoP is by far the one that have brought us the most. Ofc they will just keep getting better no doubt.
MoP is really great, but i think the Lore of Wotlk is the best, the whole feeling with undead and the Lich King was just chilling, thrilling and awesome! Hope we will get a new nasty 'bad guy'. :P
In my opinion the first thing that makes people choosing their best Expansion is the Story
Alot of people love Illidan and demons, i personally loved more Lich King story and the Undead
for some Karazhan was the best raid, for others was Ulduar
Mop is a great expansion, the wrong things is to much Dailys (at start) and for many not to much engaging story, at least not to the point of Illidan and Arthas
Now Ashara? maybe, the return of the Burning Legion? even better
so in the ends all depends if the story is captivating enough for people to return to the game (or stay)