At the end of the day, it comes down to your own playstyle and preference, and that dictates what's easiest to play. However, from my experience, I'd say Warrior is the easiest because you don't have to worry too much about positioning (as you do when playing a ranged class like Mage or Hunter), you can take some punishment without dying instantly (as would happen when playing Assassin), and you can still dish out a fair amount of damage (as you can't typically do as a Guardian).