Pretty much all classes take little time to learn/master. There isn't enough variety in the game (with regards to skills, and combos), so every class generally follows a set of actions. 1vs1's end up being damage output, reaction speed, and at times class (ex. a good mage will always beat a good wb).
Easiest class to learn and play well? WF. Straight forward combo, can tank very well, can do insane damage, debuffs, dark ironwood, etc.
Would like to say sin as well, and probably would've before I made a 3.33 sin and played it for a week (did fairly well as an experienced player, but it's not an easy class).
Can't really think of any other class that's insanely easy to play in 1 vs 1, though an endgame mage, DD ep, and ea are fairly easy if you're a decent player overall and have decent timing.
Haven't played mystic, psy, or seeker so no comment on those.
