For most of my WoW career, my main character and first love was a druid. I particularly liked the balance spec even when it wasn't fashionable to do so. I still like my druid, but she is not even close to my favorite character. I've leveled paladins and mages, priests and shamans. I've toyed with hunters and warlocks. But the class that I love the best now is most definitely the monk.
I haven't felt this way towards a class in a game since my bard in Everquest. Well, that's true for fantasy characters. I also adore my operative in Star Wars: The Old Republic. But for World of Warcraft, my true love is the monk. I played with the brewmaster a bit, but while I like the spec, I'm not overly fond of tanking, so it's not quite for me. It's easier to do solo questing and dailies on my windwalker so that is the spec I play the most, but my favorite is the mistweaver. I enjoy the different healing mechanics of the monk and I feel I do a good job with them.