Like many others, I thoroughly enjoyed BC. The raids were amazing, and it was also my first MMO expansion I was ever part of. So it has a special place in my heart!
My favorite expansion was Wrath of the Lich King by far, that was really the peak of WoW, but I can't really tell you why though. I'm liking Warlords, but I don't think the dungeons are as good.
I think they all have been done really well except for Catacysm. But that is because I had 10 maxed character so had no reason to do most of the 1-60 content that was added with that expansion. WOTLK probably is my favorite expansion because it added LFD which allow casual players to actually do dungeons frequently, and allowed me to experiment with tanking, which is now my main role. In TBC and Vanilla, 5 man content was not really accessible without a lot of time invested and it was hard to jump in tanking.
I Still like frozen Throne, call me old fashioned call me weak But i loved that expansion to a new era of WoW, it brought so much to us like DotA haha I think maybe that is why i like it the most. it gave us something other than WoW with all the things WOW had to offer.
I do think Burning Crusade was a fantastic expansion, but I think it rode the coat-tails of how fantastic the original game was. A lot of bloat set in between BC and Draenor. Draenor has to be my favorite because I feel like it actually improves on the classic formula. There are linear quest lines, but are rewarded for exploring the nooks and crannies of zones. In addition, the art style feels like the traditional game again too. I'm not sure if other people noticed, but the art style after Burning Crusade, as it became higher resolution, became noisier. The painterly style of the classic game was cleaner, and they went back to that in Draenor as well. I could probably ramble on all day about Draenor, haha. Honestly, the best expansion is probably the one where you played with the best group of people.
Probably BC. It was the first one I ever brought and now that I have all of them it is still my favorite, none of the others just gave me the same experience as BC