Karlach as a Barbarian can use almost anything really.
Knife of the Undermountain King can be decent for Barbarian, but a little later in the game. As you can get some gloves that give your off-hand weapon damage scaling from your attribute (Something Barbarian lacks from their class unlike Fighter or Ranger), throw in Barbarian level 9 bonus of Savage Criticals for extra dice on crits and some other Act 2-3 gear for more crit range and Barbarian's Reckless Attack for advantage and she can be a crit machine.
Of course, Wyll and Astarion can both utilize the weapon too. Wyll with Pact of the Blade to gain proficiency with it, plus Spell Sniper feat can pump out some critical Eldritch Blasts. While Astarion can use a bunch of gear available in Act 2 that provides bonus critical range when obscured (Along with a ring that gives immunity to Blind allows him to sit inside a Fog Cloud attacking with advantage with high crit chance)
There's a lot of flexibility for the weapon. Heck, you can even run it on Sorc if your race gave you proficiency with some other later game items to back it up.
In the end, the best weapon is the one you build for. There's tons of options available.
