According to most testers it's Prospector/Wendigo(on full orbs).It would seem that those 2 are the highest dps weapons in game.
I didn't compare Xeno purposefully to them since I don't think the goal for buffing of Xeno should be to make it OP or the best but to be at least one of the 10 top so people can use it as a viable heavy weapon for pve. :P
Cheers,
Atomlad