What weapons are "best" probably depends on what you're after.
In 3e rules there's no way dual-wielding hammers, flails or other low damage dice weapons would produce more than damage than two-handers. BG differs in so far as iirc you don't get that 1.5x damage bonus of using a weapon two handed. Furthermore all damage resistances are percentages instead of flat DR values, which further diminishes two-handed damage vs one-handed.
In BG1 Ashideena will produce 6.5 dmg on average (ignoring STR bonuses because they're identical anyways for all melee weapons) vs Spiderbane 7.5. Not that far apart, although Spiderbane has the good kind of Freedom, the one that doesn't disturb haste, a property that's good to have.
The venom dagger is great for alpha-strikes vs pesky mages, just sneak up, backstab, and when the hit connects not only will the mage be severely injured but most likely poisoned as well and the poison ticks will interrupt his spell-casting too.
So, other than Drizzts stuff and some bows for Coran, these were the 3 most notable weapons in BG1 as far as I remember.
As far as dual-wielding is concerned in principle: whether it is worth it depends. On low levels when THAC0 is still relevant dual-wielding might actually result in you dealing less damage. Later on at higher levels when making your hits connect is no longer a concern, extra attacks certainly won't hurt, although the off-hand usually contributes little in terms of damage, the most notable contribution is usually some secondary effect, like STR 25 or another attack with the main hand like in the case of Belm. So if you're after that secondary effect, dual-wielding is a good way to get it.
However, experience while playing the game, dual-wielding, single-hand wielding with and without shield and using a two-handed weapon I came to conclude that using two-handed weapons is the best - for me - because
1) they're readily available, right from the beginning of BG1
2) vorpal effects from the githyanki sword, poison damage from Gram, the water-draining from the Wave all that can add up real quick especially with a lot of ApR and contributes to killing mobs quickly, quicker than dual-wielding any weapons in my experience.
Finally, I never liked the flail of ages because the freedom of movement prevents haste. At least that was how it always was on my installments, Spiderbane, Keldorns armor didn't prevent haste, FoA did. So, for me that means it's right out of the window.
I'd rather make them save vs death 10 times a round with the silver blade. Debuff them with malison, see them die left and right. The wave and the ravager halberds are also extremely good, it's just that good halberds aren't as readily and consistently available as swords.
Likewise, I won't start using bastard swords just because in ToB suddenly a good one appears. Anyway, especially in BG2 there are lots of strong weapons and one probably can make a case for most of them.