Thats up for debate... and until the endgame is here, I don't think we'll know.
In melee Assassins have the highest single hits, followed by Rangers, Swashy's and Brigands, then Dirges/Troubadours. This is however countered by timers on the different class's. The big hitting assassin abilities are on long timers, Assassins are usually top dog in the first fight, and Rangers usually out pace them over the course of several fights.
Swashys and Brigands are very good damage dealers (just under assassins and rangers), and have more utility than the Assassins and Rangers.
Bards, while being the lower end of the Scout DPS ladder, assists in raising the whole groups DPS on a whole, and are therefore very nice.
Outside of Melee the Wizard/Warlock class's offer up some exellent DPS as well, but comes more in a burst form.
(I copy and pasted this from my response to a similar question asked about a month or 2 ago, editted to reflect some minor changes in how much we know)