[SpongeBobTheDestoyer]

Take your pick. It might help if you state whether you're looking for something more defensive or just a plain damage dealer. The pure classed kensai is going to add plenty of damage on his own so maybe you could look for something with some defensive properties.

Upgraded Flail of Ages - offensive masher
Runehammer - I think its negative plane protected
Arguvandale(am I close) - it increases strength once per day
Hindo's Doom - Restoration and then later Greater Restoration, nice defensive katana

You could combine one of those with something brought along from SOA, unless you're starting out in TOB. Or go with one of the most overpowered weapons in the game, Ravager.