It's definately a matter of trade offs. My character is a lvl., 12 Kensai, so he can't wear armor or use shields, but he gets incredible attack bonuses. He's maxed out in Single Wopons proficiency, Grand Master 2HS.
He is devestating with the Vorpal Blade. Perhaps it is the Kensai's attack bonuses but he gets those failed saves against death for instant kills very frequently. I had one case where 3 targets in a row died instant deaths.
My Kensai has toasted 2 Adamantine Golems all by himself with the Vorpal blade in quick order, and he slayed The Big Red "Kraag" Machine on the second hit.

I am obviously biased toward the Vorpal Blade...

I cannot pass up the AC for my main character so he has to carry a shield. But Minsc is a deadly one with the 2Handers. He is also good with the Long Bow. He is a damn fine NPC. Tangent sorry.

In your opinion what is the best sword for a fighter character to use in the game?

-Draelren

Do you mean what's the best sword type (i.e. 2 handed, longsword, bastard sword, etc...)?

OR

What's the best specific sword (i.e. Daystar, Holy Avenger, Vorpal, Celestial Fury, etc.)?

Best specific sword, regardless of type (ie Vorpal, etc...)

-Draelren

If you're a Paladin ... The Holy Avenger!!!
+5 2handed, 50% Magic resistance, dispels magic on any hit!!! Only paladins need apply.

After that it greatly depends upon whether you like carrying a shield or not. There are some other great 2 handed swords...

Lilacor +3 and talks to you...
Warblade +4
Vorpal +3 + 15% chance of death on any hit (save vs death)
There's one other +4 2h SW that only non-good char's can carry. (Don't remember the name)

If you like shields or dual wielding,
Celestial Fury (katana) +3, chance of stunning on any hit, plus other special abilities... A truly GREAT weapon!!!
Daystar +2(long sword), +4 vs evil, 2x damage to undead creates, cast sunray 1xday.
The Equalizer +3 (longsw), thac0 and damage plusses depend on alignment of target Further from nuetral, the greater the plusses.

A lot of it is personal opinion. I happen to prefer driving my AC as low as possible and like carrying shields, so my characters tend to carry 1h swords.

However, it's stated in a few places that 2handed weapons can carry more enhantment than 1 handed weapons and therefore be more powerful.

I'd say that the 2handed swords are the most powerful weapons, but at the cost of some AC.

Hope this helps some.

I agree with Magness. (Great name. I have been a Paladin named Magnus for almost 20 years) To the topic... 2-handed swords are extremely powerful, but I like the ability to carry an awesome shield and dole out good damage. That said, I am currently wielding the HA and carrying a +3 long bow in BG2. It is just too powerful to pass up. Sometimes you just have to adjust.
BTW, A nice plus for 2-handed swords is the ability to have a bow in your quick weapon slot

There's not a lot of great longswords in BG2. The best one is not available till the end of the game so it's essentially useless (like being given a Ferrari when you're 80). The pick of the bunch is probably the Equalizer or the Dragonslayer for dragons.

2H swords would be Vorpal and Carsomyr. Warblade is good too. There are much better 2H swords than longswords in the game. Soul Reaver +4 is great for evil chars.

Katanas: Celestial Fury, the second best weapon in the game after Crom Faeyr.

The.Relic, I noticed that you are spending your point at Single Weapons(it means only 1-handed weapon) proficiency
while you are using 2HS, because you wont get any bonuses for that.
Instead you should use your points at 2-Handed Weapons proficiency, then
you'll get your bonuses. Just a suggestion

Heh, done both actually, I left out 2HS, working on SH so I can use my Flail of Ages and DragonSlayer.
Sorry about the typo...