As far as best overall feel I might give it to Wrath. But like he said, it got old and gloomy fast. If I had to pick one permanently I would go with BC. Vanilla had a good feel, as it was a vanilla feel. The world felt much more natural and cohesive, rather than forcing 1 sort of atmosphere down your throat.
As far as MoP, I was sick of it before I hit 86.