Okay, you are asking 3 separate questions in your poll:

Q: What is the best class to get to level 60?
A: This is a difficult, subjective, question. What are you going to do at level 60? Are you just going to stop at 60 and play the old world content? Might reconsider this as you will be hard pressed to find much to do solo. If it's 'level to 80' then whichever class you want to play at 80. If you're rerolling DK then stop at 55. If you're PVPing then stop at 59.

Q: What is the easiest class to get to level 60?
Solo questing, Hunter or Warlock are probably tops.

Q: What is the 'funnest' class to get to level 60?
A: Whichever you enjoy playing most!