The most fun I've had was playing 4x mages & a priest. Stuff just disapears, no tank needed until outlands.
4x pom pyro is just evil. Instant Arcane explosion, & blastwave are quite fun as well.

Once they hit 70, they didn't quite scale out the way I thought they would.

Playing shammies now. Much less fun to level & the "OMGWTFBBQ" isnt quite as tasty as it was on the mages, but I hear that changes
as you hit 70 & get geared , which was the opposite of what happened with me & the mages.

Overall I think it boils down to what you like to play, & I like to play mages.