For melee, I think it has to be flanking. First Strike and Relentless are also very nice, but don't always trigger. Bladestorm seems to always work when its conditions are met - but if you don't deal the killing blow when another opponent is adjacent, then it cannot trigger (Double Tap is more useful since there only has to be another opponent in range). Flanking is the best since, although it is conditional, with some experience you can set up that condition most turns and it always triggers when you do. My Swashbukler's Sarah Steele has an additional flanking attack about four turns out of five.