It's hard to say what items will be added to BG:EE and its sequels in the future. So if you want to powergame, here are the base stats for each weapon.
Below there are the damage dice, and then in parentheses is the weapon speed, which only matters in determining at what point in the round you attack -- it doesn't grant you any extra attacks or anything like that. But if you kill a guy before he attacks for that round, that's always nice. A round is 6 seconds. A lower number here is faster.
Weapon mastery (4 pips) gives a -1 to speed factor. Grand mastery (5 pips) gives -3. Additionally, for each level of enchantment, -1 is applied to the speed factor. This means a fighter with a grand mastery in bastard swords swings a +5 bastard sword with a speed factor of 0.
Note that Katanas, Wakizashis and Ninja-Tos are rarer finds than other kinds of weapons (especially true for Katanas), so you might have to wait awhile to find one.
And as you get better and better enchantments on weapons, these stats become a smaller and smaller portion of the damage.
EDIT: Backstabs work with long sword, short sword, katana, scimitar, dagger, club and quarterstaff -- the melee weapons usable by thieves.
The damage type matters because some types of damage have an easier or harder time hitting certain types of armor.
Slashing gets a penalty to studded (-2), chain (-2), plate (-3) and full plate (-4) armor.
Piercing gets a bonus to leather armor (+2) and a penalty to studded (-1), splint (-1) and full plate (-3) armor.
Bludgeoning gets a bonus to chain armor (+2) and a penalty to splint armor (-2).
One-handers:Slashing: Katana: 1d10 (4)
Bastard Sword: 2d4 (8)
Battle Axe: 1d8 (7)
Scimitar: 1d8 (5)
Ninja-To: 1d8 (4)
Long Sword: 1d8 (5)
Piercing: Wakizashi: 1d8 (3)
Short Sword: 1d6 (3)
Dagger: 1d4 (2)
Bludgeoning: Morning Star: 2d4 (7)
Flail: 1d6 + 1 (7)
Mace: 1d6 + 1 (7)
Club: 1d6 (4)
Warhammer: 1d4 + 1 (4)
Two-handers:Two-handed sword: 1d10 (slashing) (10)
Halberd: 1d10 (piercing) (9)
Spear: 1d6 (piercing) (6)
Quarterstaff: 1d6 (bludgeoning) (4)
Missile Weapons (assuming normal ammo; all piercing):Heavy Crossbow: 1d8 + 2 (10)
Light Crossbow: 1d8 (5)
Throwing Axe: 1d6 + 1 (4)
Composite Long Bow: 1d6 + 1, +1 THAC0 (7)
Long Bow: 1d6, +1 THAC0 (7)
Short Bow: 1d6 (6)
Sling: 1d4 + 1 (6)
Throwing Dagger: 1d4 (2)
Dart: 1d3 (2)
• Scimitar/Wakizashi/Ninja-To is a single proficiency.
• Flail/Morning Star is a single proficiency.
• Light and Heavy Crossbows use the Crossbow proficiency.
• Composite Long Bow uses the Long Bow proficiency.
• Battle and Throwing Axes use the Axe proficiency.
• Daggers and Throwing Daggers use the Dagger proficiency.