TBC > WotLK > MoP > Cata. (started at Vanilla release btw)
TBC for me is rose tinted glasses, but not because of nostalgia (I never liked nostalgia anyway). It's because of the overall theme. It feels like the most inspired expansion of all.
The mix of outer space, flowing skies, huge demons, gloomy giant mushrooms, energy beams, big floral domes, having to fly into the void to find a dungeon entrance, roaring robots creating an earthquake each time they step into the ground, the bewitching and peaceful oriental music in Shattrah, demon meteors showering in the background, booming into your ears as they crashed onto the Shadowmoon Valley, Quel' Danas isle being built by the whole server among crazy improvized PvP ... and of course, a real sense of progression for your main, gated content that could be unlocked with a bit of guild commitment.
All of this felt like a strange, beautiful dream, that I will never be able to get out of my head as much as other expansions.
I would give anything to live such an experience again.