Bard, summoner or ninja.

And that is from a " easy enough to be not totally crap w/o trying hard" perspective. Those classes are not terribly difficult or complex to not be atrocious on.

But don't think for 1 second that any dps is "easy to do good dps" on. Every dps is incredibly difficult to min max on, and sure those 3 probably are the easiest, but lol if "easy" is the right word here.