The worst game(not sure you can call it expansion) was by far Vanilla. Then Cata. Then TBC, Then MoP, Then Wrath
Keep in mind I have probably been playing as long as anybody, started when game came out, sure I have taken some breaks here and there.
For the most part I am guessing the people that like Vanilla either
-Never played Vanilla just heard how great it was
-Didn't have a life, and only played WoW, because truthfully, you could only play WoW if you played Vanilla, considering it took 1 year to go from level 53-54.(Kidding)
-Forgot what the game play was actually like and how broke it was
-Forgot what a time sink the game was, rewarded no skill, the philosophy back then was, play a lot and get rewarded. That's it.
Mostly the game has improved each expansion some cool thinks they have added.