There are no difficulties for dps classes when it comes to PvE, really. You just keep your rotation, what's the big deal? Every class has its own thing, yeah, but that's not what difficult is. Now when it comes to PvP, hardest two are subtlety rogue and frost mage. Rogue because you need to use everything your class has in order to win and frost mage because of the arsenal of spells. Once you get used to your shortcuts, it's all easy no matter which class you play. And the more you play it, the faster you do stuff. When you think of it, it all really is about speed.
Easiest classes to play are the ones with damage over time spells. Keep them up all the time, wow so hard, help me!
No class is 'hard', that's a bad word. Same with 'best', there should be no room for either one of these two in World of Warcraft.