The Morning Star. It tends to show up a lot with holy warrior characters such as clerics and paladins, and I feel it would fit in with the style of the game.
It might deal a little less damage than other weapon types in its class, but it has the ability to crush/penetrate a small amount of armor, a natural counterpoint to shield/katana. A disciple's rod or simple club as starting weapon, flanged mace in class 2, and morning star in class 4.
Having armor penetration as a starting weapon is almost completely useless. You get a slight damage bonus against crabs. Maybe it could do a small amount of the damage dealt to the target at AOE on adjacent tiles? Or maybe just the 3 tiles behind the enemy.