MoP=Legion>WoTLK>TBC>Vanilla>Cata>WoD.
MoP has been pretty great in terms of the content it brought, and the raids were pretty awesome. Had lots of fun with CMs too, as well as the small patches that brought in new content.
So far, Legion is doing great as well. I'm having tons of fun with its content. TBC and Vanilla make it to the middle of the list because, once the novelty wears off, you realize the asinine requirements they impose on you to actually play the freaking game. I'm sorry, but I don't like to guild swap in order to raid the latest content in TBC. In Vanilla, it was even worse. Many of today's players wouldn't even stand the grind for raiding mats, let alone the requirements to be in a decent guild to see the raiding content.