ToV is certainly MH BiS dagger (highest weapon damage). There are a few daggers that could be argued for the offhand. Goblin screwdriver has 5 agility, so its best in terms of pure attack power. Assassin's blade and tailspike sacrifice 1 agility for 3 crit, which could be argued for as well.
For subtlety, a dagger doesn't have to be used offhand, allowing for shadowfang, thief's blade, and blackwater cutlass to be possible offhands.