MoP > WotLK > BC > Cata.
That's my opinion on the whole matter.
Now to wait for another thread like this to show up to share my opinion once more
I can't really quantify anything to prove why one expansion is better, I just know that so far I've been enjoying my time playing MoP more than I was in the first few months of any other expansion.
In particular I'm enjoying the raids more so than anything else.