Dirty Fighting ftw!?!?! Why? 'cause it's the coolest. Fencing is just pompous, and florentine is for girls! As soon as i saw my first fencing graphic, i grabbed a respec card and ate it. Needed gravy.
Dirty fighting is the best purely because everyone says it sucks i think. Besides, tanth acts about 90% Pirate, so it's only fitting really.
On a serious note though, for boarding combat, i'd say dirty fighting everytime, if you use:
Terrifying shot, and the other 2 mezzes that i forget the name of, and his garuanteed Bleed normal attack.
Dive onto a ship, mezz 3, and then just cycle targets bleeding everyone. In PvP Boarding, dirty fighting is like the shock+awe tactics. Balls to all this running around the ship, get my NPC's to fight his NPC's, i slice and dice whoever's nearest. In the end there's just Mr. Florentine left, crying in the corner as 6 grinning marines run at him.
Boarding combat is best done the same way you kill a circus: Go for the juggler. (read: Jugular, for those who don't get it)
For dueling? any but Dirty fighting, no doubt =)
99% personal preference really, same as with "what's the best ship"