Definitely MoP! Gotta love all the immature idiots who missed out because LoLPandas.
Wrath was a very BIG expansion, huge zones, large raids, lots of players, but many aspects of it were lackluster. I think TBC was the better expansion among the first three. For a great fan of Warcraft III, the most glaring issue with Wrath was its failure to spark my RTS nostalgia, in spite of featuring the most prominent character from those games. It's like they designed everything without ever looking back. Pandaria succeeded on the nostalgia front even though it was an entirely new continent with a ton of new lore, so it gets major props.
Legion is a very successful expansion in that it keeps people playing more, but a lot of that is due to content gating and longer grinds, which is not so great. It's still a pretty decent expansion though.