Let's just go with 10 being the best one, 1 being the worst, rather than a flat out rating. Also objectively speaking:
That all being said, I had the most fun in Vanilla and TBC. My guild died down towards the latter half of Wrath, so I did have a lot of fun there too up until the end. Cata is the reason I didn't play the game for over a year, because MoP is really good. I'm actually looking forward to the next xpac as I think the team is finally figuring it out.