Honestly, it depends on your class and battle strategy. I personally have Repel Boarders, Flanking, Riposte, and Relentless 2 on mine, as well as a boost in Will for my Catbeard since that would help in Relentless as well as his damage.
If you try to set up flanking opportunities, Catbeard does great with Riposte 2, Relentless 2, Flanking, Dodge 4, Accuracy 4, Rough 4, and Will 4. If you prefer to shield your squishies with melee units, Riposte 3 keeps Catbeard alive longer and gives him more ripostes and therefore more relentless triggers against melee units. Since flanking cues more enemy attacks and a successful flank attack triggers relentless as well, I can frequently get 8 attacks against melee enemies or 4-5 attacks against ranged units in a round with the previous build.