should be a +10 jap katara (something of sorrow) with 1 star. make sure the jap katara isnt below average in stats while clean.
dbs dont need +1 atk speed since most daggers r fast 4 speed, dbs can reach speed 0 with booster+dsi+green pot. only other endgame dagger worth considering is sw dagger which is 3 speed, therefore u can reach speed 0 with booster+green pot. 50% buff duration is more helpful for permanent final cut, and less buffing while training, bossing , etc.
Go for buff duration IA, 25%+ should be fine given you got mechanic card & remember to spamm SR when possible. For xposing, the higher base attack = the better obviously, so try getting Japanese katara. Those cannot be crafted sadly as no recipe exists, but they do drop ready from ranmaru.