I'm going to put down Wrath because of the storyline that was playing out and the resolution of a major part of Warcraft III's story, and its huge advancements beyond TBC and Vanilla. Cataclysm continued those advancements but they were incremental beyond what Wrath delivered, and while it was great at the start I feel like they missed an opportunity to explore the character of Deathwing and make DS more satisfying (it wasn't awful like a lot of people say, some good fights some bad, just wasn't amazing like an end of xpac raid kind of should be... it was just another raid. Spine of Deathwing was great though).
MoP has shown a huge step forward from Cata, adding so much new content I damn near killed myself trying to do half of it. And a lot of the little touches here and there have been awesome. Maybe by the end I'll actually say it's MoP.
---------- Post added 2013-03-25 at 02:12 AM ----------
Eh, people only remember an expansion based on the last raid, they forget a lot of what Cata did right. Personally I got my money's worth before I even got to level 85 just based on the old world revamp, which was amazing. Also LFR and transmog were great features (whatever people might say about LFR). The launch heroics were freaking awesome as well. ZG and ZA were also great the first few times - though it was a mistake to split the queues because doing them 7 times a week was horrible.