A fair number of foes in IWD are slash/pierce resistant so you'll definitely want flails or maces. There are also a large number of high-quality longswords so they're a solid choice as well. Very few katanas however and none are in the same league as Celestial Fury so that wouldn't be a recommended weapon choice.
Seconded, flails and longswords are great. They work well for all fighter classes. Daggers often have mage bonuses and hammers cleric bonuses. There are some good axes and two handed swords too. There is one late game mace that is brilliant vs undead
Due to loot randomisation you may want to check a walkthrough so you don’t miss the best options if you are tied to one weapon type
While not quite the kensai you're asking about, my dad and I played a multiplayer game where my dual-wielding paladin ended up being the party tank. She was using dual longswords for her main weapons and then I had a set of flails for when I needed bludgeoning damage. She was a blast to play!
Comments
Due to loot randomisation you may want to check a walkthrough so you don’t miss the best options if you are tied to one weapon type