For a PvP realm i would go for a rogue for the pure fact that you can gank people around your level, and avoid those that would gank you. They aren't the funnest to level as its a very solo mob orientated class.
As has been said, hunters are the easiest, but funnest? I don't think so. Its a bot class for the reason it is so simple to play.
I found leveling a mage rather fun, and leveling a druid is fun once you've gotten past the suckish levels at the start. If you have the motivation to continue leveling i would go for a druid