WOTLK for me.
MOP is cool, but it doesn't feel like an expansion to me. Nobody really hangs out in the MOP zones, and we are almost done with the MOP continent already.
Not to mention, I feel the raids are awful. There is little to no lore, you can't explore the areas, the art isn't good, there is no really cool unique audio work, there is no epic intro or cut scenes. It is boss, boss, trash, boss and repeat. All pretty cookie cutter. Even Lei Shen, who is the last boss of the MOP continent. Is that Ulduar quality, the second to last raid in WOTLK? Is it ICC quality? The answer to both is no, yet it's the best raid in this expansion.
What about the "main city" or area of the expansion. Does it compare to Dalaran?
Don't get me wrong, I like MOP a lot, I just feel WOTLK is in a tier of it's own. Ulduar alone gets it the title.