If you want the most common one: Longsword.
When wielding two weapons, take care with off-hand penalties. Always ensure you have Ambidexterity & Two Weapon Fighting, and later one Improved Two Weapon Fighting. Without those, don't even think about using two weapons, or double weapons. Your "to hit" will always be less when dualwielding. Even more if you wield a medium weapon (katana, for example) in your offhand.
Twobladed sword allows for a 1d8 damage weapon in the offhand, without the penalties of a medium weapon. Still need all the feats.
Scythe is a twohanded weapon, so you get 1.5 times your STR mod on damage, and it does x4 damage on a crit.
Since Weaponmasters revolve around crits, let's look at those.
19-20/x2 on a weapon means that if you roll a 19 or 20 on a 20sided dice (and you hit the opponent and then succeed on another roll against his AC), you do x2 damage. 20/x4 means that x4 damage happens on the roll of a 20.
Personally, I'd take Scimitars or dual Kukri's for a Weapon Master, and go STR all the way: they have huge critical ranges (18-20/x2), which the Weapon Master turns into 15-20/x3. Don't worry about their low base damage, as crits multiply your weapon bonus (+ 5, for example) and STR bonus too.
If you don't know the D&D rules that NWN uses, check for the NWN Manual on your disc, or on
http://www.replacementdocs.com
