I have a confession to make. When it comes to World of Warcraft, I'm pretty easy to please. Every time a new expansion comes out, that's my favorite one. Even and especially Cataclysm. I know it's not a popular opinion, but I loved the old world revamp and going through the questlines like Silverpine Forest still makes me happy.
So my current favorite expansion is Mists of Pandaria. I love the monk class. I love the pandaren. I love the epic quest lines and the farm and pet battles and ... the list goes on. Though the Brawler's Guild isn't my cup of tea, I love watching The Spousal Unit battle it out against the various contenders.