This has been done before but not for a while (plus there seem to be a fair few foke fairly new to Fallout and Fallout 2). Based on a single weapon, what are the most effective late game builds in both games?
Fallout
Power Fist
Surprising? With an agility of 10, Fast Shot, Bonus HtH Attacks and eventually Slayer as essentials. Power Fist attacks cost 1 AP (the Super Sledgehammer is a better weapon damage wise, however I don't think Bonus HtH works on melee weapons and the knockback attribute gets annoying). Optional extras include: Better Criticals, (2) Action Boy(s) and (2) Bonus Move. Actually, provided Agility is good, Bonus Move > Action Boy for this. Overall this can mean 12 instant critical hits, i.e. you can wipe the floor with anything.
Minigun (and normally to a lesser extent, the Gattling Laser)
Impractical, as it requires the Sniper perk - takes ages to get in Fallout. However Fast Shot + Bonus Rate of Fire + Action Boy (2) + Sniper (with luck >7) + Better Criticals = carnage. Bonus ranged damage is very good here as well. There are lots of perk permutations in this build, but providing you have good luck, I think this tops it in terms of damage administration. Not saying it's practical though. You can fire thrice in the first round of combat.
Turbo Plasma Rifle
Much the same setup as for minigun, only you can potentially fire it 6 times - a lot better than the F2 version. Same for the Alien Blaster, which does more damage but at the expense of having a terrible range. Generally inferior to the first two in terms of damage, easier and more practical to play though.
Fallout 2
Unarmed weapons cannot become faster than 2 AP to use, which makes them all distinctly inferior to ranged weapons. Also, stuff is better at killing you in F2 - making HtH harder (though with Prizefighter and power armour it is very possible to tank the whole game).
Vindicator minigun/ Avenger/ Bozar
As with minigun in F1, though getting Sniper in a normal game is worlds more realistic, making this build a lot more likely. This can do some brutal damage, and if used correctly The Enclave + Navarro becomes a doddle, even on the hardest difficulties. The availability of ammunition makes this a difficult weapon of choice to use often though. The Avenger minigun and Bozar become far more practical. The Avenger is the Vindicators equal against some opponents (if you invest in bonus ranged damage anyway), however it still lacks a bit of a punch (relatively speaking - it doesn't matter a whole lot with Sniper) against power armour. The Bozar uses cheap ammunition with great modifiers, fires less bullets and still nails virtually everything, making it the most practical of the three. Nothing comes close to competing with this damage wise.
Gauss Pistol (and rifle)
When you have Sniper and good luck, this seems superior to the rifle in my eyes. 6, ludicrously accurate, high-probability-of-critical hits, within a massive range. Armour (apart from the Boss'), provides little relief.
Pulse Rifle
Distinctly superior to the turbo plasma rifle, basically a slow alien blaster with much better range. Lots of damage, and again most armour won't be a problem. Still seems inferior to the Gauss Pistol though.
End Boss Plasma Gun
Of no consequence, seeing as you have completed the game. Had to include it though, I mean it'd basically a turbo plasma rifle with burst capability
Other notable weapons:-
F1
1) .223 Pistol - best small gun in the original, decent range, good damage, owns most armour, style.
2) RR LE BB Gun - inferior to 1), but deserves a mention. Fires quick, massive clip, good damage, not so great armour penetration. Shooting children and Gizmo with it are fun past times.
3) Hunting Rifle - with a refined ability to land bullets in peoples eyes, one can finish the game with this no problems. Unless you have 12 AP, this is better than the Sniper rifle IMO.
4) Death Satchel - Get a bag from Junktown and store all the spare explosives in it. Activate detonators, drop next to target, run. Bang.
F2
1) Those from F1, although they are eclipsed by better weapons - Gauss weaponry is miles ahead of other small guns really.
2) FN FAL HPFA - strange and fairly uncommon assault rifle, as a single shot weapon, it's superior to all of the assault rifles except the H&K G11(E) and (maybe?) the XL70E3. Does have one feature that (arguably) sets it above all other (non gauss) small guns though, that being the bullets fired per burst - 20. It has a faster rate of fire than all other bullet weapons bar miniguns (even the HK P90c and Bozar!). It's ammo has the same DR mod as the 4.7mm used by the H&K G11, though a weaker damage mod. 5mm ammo, as used by the XL70E3, has weak mods in comparison. Because if this I'd say that the FN FAL HPFA is up there with the H&K G11E as the best assault rifle (don't recall if the later is technically an SMG, don't think it is), it's also superior to all SMG's and shotguns (Pancor Jackhammer is still a very good gun though). Small guns running on .223 or 2mm are it's only out right superiors.
3) Needler Pistol - Actually a decent gun if you can get the AP ammo (which isn't really AP, just double damage).
4) Robot rocket launcher - For a rocket launcher, it has poor damage. It also requires only 3 AP and has a 'clip' of 6, think Fast Shot + Bonus Rate of Fire + Sniper....evil laugh. A rather exoensive AP rocket habit though.
Just my two pence, be interested to here peoples opinions, especially with regards to the FN FAL HPFA, which I only realized was decent on a recent play through. I feel that assault rifles are underrated by some, not in F1 as there is only one and it's poor-mediocre for what it is. In F2 there is a bunch, of which the FN FAL HPFA, XL70E3, H&K G11 and H&K G11E are all amongst the better small guns around.
Small Guns Rankings
Top Class: Gauss Pistol, gauss rifle.
Second class: H&K G11E, FN FAL HPFA, .223 pistol.
Third class: H&K G11, needler pistol, pancor jackhammer, HK P90c, sniper rifle, scoped hunting rifle.
Fourth class: H&K CAWS, combat shotgun (unquestionably inferior, though by a small margin), grease gun, FN FAL (with nighsight), .44 magnum (with speedloader), RR LE BB Gun.
You could probably argue for the .223 rifles, pancor and LE BB gun to be moved up a class in many circumstances.
Fallout
Power Fist
Surprising? With an agility of 10, Fast Shot, Bonus HtH Attacks and eventually Slayer as essentials. Power Fist attacks cost 1 AP (the Super Sledgehammer is a better weapon damage wise, however I don't think Bonus HtH works on melee weapons and the knockback attribute gets annoying). Optional extras include: Better Criticals, (2) Action Boy(s) and (2) Bonus Move. Actually, provided Agility is good, Bonus Move > Action Boy for this. Overall this can mean 12 instant critical hits, i.e. you can wipe the floor with anything.
Minigun (and normally to a lesser extent, the Gattling Laser)
Impractical, as it requires the Sniper perk - takes ages to get in Fallout. However Fast Shot + Bonus Rate of Fire + Action Boy (2) + Sniper (with luck >7) + Better Criticals = carnage. Bonus ranged damage is very good here as well. There are lots of perk permutations in this build, but providing you have good luck, I think this tops it in terms of damage administration. Not saying it's practical though. You can fire thrice in the first round of combat.
Turbo Plasma Rifle
Much the same setup as for minigun, only you can potentially fire it 6 times - a lot better than the F2 version. Same for the Alien Blaster, which does more damage but at the expense of having a terrible range. Generally inferior to the first two in terms of damage, easier and more practical to play though.
Fallout 2
Unarmed weapons cannot become faster than 2 AP to use, which makes them all distinctly inferior to ranged weapons. Also, stuff is better at killing you in F2 - making HtH harder (though with Prizefighter and power armour it is very possible to tank the whole game).
Vindicator minigun/ Avenger/ Bozar
As with minigun in F1, though getting Sniper in a normal game is worlds more realistic, making this build a lot more likely. This can do some brutal damage, and if used correctly The Enclave + Navarro becomes a doddle, even on the hardest difficulties. The availability of ammunition makes this a difficult weapon of choice to use often though. The Avenger minigun and Bozar become far more practical. The Avenger is the Vindicators equal against some opponents (if you invest in bonus ranged damage anyway), however it still lacks a bit of a punch (relatively speaking - it doesn't matter a whole lot with Sniper) against power armour. The Bozar uses cheap ammunition with great modifiers, fires less bullets and still nails virtually everything, making it the most practical of the three. Nothing comes close to competing with this damage wise.
Gauss Pistol (and rifle)
When you have Sniper and good luck, this seems superior to the rifle in my eyes. 6, ludicrously accurate, high-probability-of-critical hits, within a massive range. Armour (apart from the Boss'), provides little relief.
Pulse Rifle
Distinctly superior to the turbo plasma rifle, basically a slow alien blaster with much better range. Lots of damage, and again most armour won't be a problem. Still seems inferior to the Gauss Pistol though.
End Boss Plasma Gun
Of no consequence, seeing as you have completed the game. Had to include it though, I mean it'd basically a turbo plasma rifle with burst capability

Other notable weapons:-
F1
1) .223 Pistol - best small gun in the original, decent range, good damage, owns most armour, style.
2) RR LE BB Gun - inferior to 1), but deserves a mention. Fires quick, massive clip, good damage, not so great armour penetration. Shooting children and Gizmo with it are fun past times.
3) Hunting Rifle - with a refined ability to land bullets in peoples eyes, one can finish the game with this no problems. Unless you have 12 AP, this is better than the Sniper rifle IMO.
4) Death Satchel - Get a bag from Junktown and store all the spare explosives in it. Activate detonators, drop next to target, run. Bang.
F2
1) Those from F1, although they are eclipsed by better weapons - Gauss weaponry is miles ahead of other small guns really.
2) FN FAL HPFA - strange and fairly uncommon assault rifle, as a single shot weapon, it's superior to all of the assault rifles except the H&K G11(E) and (maybe?) the XL70E3. Does have one feature that (arguably) sets it above all other (non gauss) small guns though, that being the bullets fired per burst - 20. It has a faster rate of fire than all other bullet weapons bar miniguns (even the HK P90c and Bozar!). It's ammo has the same DR mod as the 4.7mm used by the H&K G11, though a weaker damage mod. 5mm ammo, as used by the XL70E3, has weak mods in comparison. Because if this I'd say that the FN FAL HPFA is up there with the H&K G11E as the best assault rifle (don't recall if the later is technically an SMG, don't think it is), it's also superior to all SMG's and shotguns (Pancor Jackhammer is still a very good gun though). Small guns running on .223 or 2mm are it's only out right superiors.
3) Needler Pistol - Actually a decent gun if you can get the AP ammo (which isn't really AP, just double damage).
4) Robot rocket launcher - For a rocket launcher, it has poor damage. It also requires only 3 AP and has a 'clip' of 6, think Fast Shot + Bonus Rate of Fire + Sniper....evil laugh. A rather exoensive AP rocket habit though.
Just my two pence, be interested to here peoples opinions, especially with regards to the FN FAL HPFA, which I only realized was decent on a recent play through. I feel that assault rifles are underrated by some, not in F1 as there is only one and it's poor-mediocre for what it is. In F2 there is a bunch, of which the FN FAL HPFA, XL70E3, H&K G11 and H&K G11E are all amongst the better small guns around.
Small Guns Rankings
Top Class: Gauss Pistol, gauss rifle.
Second class: H&K G11E, FN FAL HPFA, .223 pistol.
Third class: H&K G11, needler pistol, pancor jackhammer, HK P90c, sniper rifle, scoped hunting rifle.
Fourth class: H&K CAWS, combat shotgun (unquestionably inferior, though by a small margin), grease gun, FN FAL (with nighsight), .44 magnum (with speedloader), RR LE BB Gun.
You could probably argue for the .223 rifles, pancor and LE BB gun to be moved up a class in many circumstances.