I love playing my warlock and hunter in MoP. But my hearth will always be with my resto druid.
Paladin, being the right hand of gods and all that.
No better class in WOW or in D&D type lore
At this point it would probably be rogue. It used to be warlock, and then death knight, but I find both of those classes to be too simple now. Rogues at least take a little bit of finesse to play well, and that keeps it interesting. Too bad they are a nightmare to level.
Druid. The druid is a very versatile class and moonkin, restoration and feral are all very fun.