They shouldn't be the best ones, there should be choice. Different weapons all relatively equally viable for different builds. But right now, any kind of sword build, you d be crazy to take greatsword for example, instead of elven curve blade/falchion.
There is a choice: you can use an elven curve blade or a one-handed estoc if you spend a feat, or you can use a regular martial weapon and use that feat on something else. Taking an exotic weapon proficiency is a major cost, unless you're playing an archetype like a Sword Saint that gets it for free. You have to compare the feat opportunity cost. It's not estoc vs longsword, its estoc vs (longsword + one free feat).
Anything greatsword does, elven curve blade/falchion do better. The 1.5 extra damage per strike is irrelevant, since with all the extras, you will be doing 40-80 damage per strike, but the added crit range is extremely powerful.
Except for dealing damage on a normal hit, which is not nothing. If we're doing apples to apples, let's take two martial weapons so feats don't come into it, like greatsword vs falchion. Greatsword does 2-12 damage, falchion does 2-8. And 2 extra damage ain't nothing, especially when your BAB gets higher and you're doing multiple attacks per round. Let's run the numbers:
Greatsword has a 10% chance to score a critical threat, falchion has 15%. For the sake of argument, let's say half of those are confirmed criticals. In forty attacks, a greatsword will get two crits and a falchion will get 3. So from those criticals, the greatsword does an extra 4d6 + (2 x bonuses) and the falchion does 6d4 +(4 x bonuses). In terms of base damage, the falchion only does 1 point more on average with three crits than the greatsword does with two. If you have 22 strength, the falchion's bonus deals an additional 9 damage. So in 40 attacks, the falchion's expanded threat range gives you 10 more damage assuming you don't have any fancy enchantments.
But what about the other 37 attacks? If only half of your crits are confirmed, that means you're missing half the time. So the greatsword has 2 crits, 18 hits and 20 misses. That's 18 hits where the greatsword does 2 more damage, so just from those regular attacks it does 36 more damage than the falchion. 34 - 10 = 24. Without any enchantments or additional feats, the greatsword does more damage than the falchion.
What would even the odds? You'd need to enchant your weapons and boost your strength to the point where the falchion's doing 24 additional damage on that third critical hit. That's a lot of enchantment! Say you have a +8 belt of strength, that's +6 to damage. Normally I'd say enlarge person, but if we do that to both characters the greatsword will get much more of a bonus from the larger weapon (3d6 vs 2d6). So we need to stack enchantments to give the falchion 18 average damage. If you have a +3 weapon and 4d6 of elemental damage--fire, frost, acid, ice--you'll do 19 more damage. That's effectively a +7 weapon, though.
So a super enchanted falchion (I don't know if one like this even exists in the game) and some major strength buffs can make the falchion oh-so-slightly better than the greatsword once you're in the late game and you're getting lots of ridiculous loot. But even that assumes your enemy has no elemental resistances. It also assumes they have no damage resistance, which changes the equations.
Sure, you can take improved critical to double your threat range, but even then you need a lot of enchantment for the falchion to be better. And you can't get improved critical until level 8 for a fighter. Plus, suppose my greatsword guy uses that feat to take weapon specialization for +2 to damage. That's +44 over the course of 20 hits, two of them criticals.
