Fallout 3, it's a better game than the others. New Vegas is ok, but it doesn't hit the bar.
Let's discuss gameplay, perhaps the best measure of a good game.
Fallout 3 had weapons with a single ammo type. It also had armor with straight up damage reduction (ie DR of 50 lops 50% of your damage off every enemy attack.) Its crafting system was extremely limited, with no more than a handful of guns.
New Vegas has weapons with a panolopy of ammo types, many of which you have to craft. It also uses damage threshold instead of damage reduction, which means that you have to exceed the DT to do the listed damage (otherwise your DAM is reduced to a 30% minimum.) And its crafting system was massively more involved.
Why is this important?
Let's take an example:
FO3, an Enclave trooper with 25 DR. You can choose a SMG or a sniper rifle. (Let's pretend that their DPS is the same for argument's sake.) In this situation, there is no functional difference between the SMG or a sniper rifle except ammo consumption. The DPS is the same, and DR knocks a flat percentage off of every hit regardless of actual damage dealt, so the only reason to choose one or the other is personal preference, the presence of a scope, or you only having ammo for one.
This isn't very nuanced.
FNV, a Brotherhood Paladin with 25 DT. You have a SMG or a sniper rifle. In this case, two things matter - damage per second and damage per shot. If you don't exceed the DT, you're only going to do 30% of the listed damage - which means 70% is going to be blocked! This means that against highly armored targets, one weapon is more useful than the other. However, that sniper rifle's high DAM is a liability against swarms of unarmored enemies, since its damage is frontloaded. Even if the DPS is the same. Which means that both weapons have actual distinct roles that they can fill.
But that's not all, Comrade! You can switch ammo types in FNV as well! So if you're sniping something unarmored, you can switch to hollow point bullets, which do more damage but are weak vs armor. If your machine gun is dealing with heavily armored targets, you can swap in AP rounds to help with that, negating 15-20 points of armor at the cost of a modest damage reduction.
And then there's handloaded rounds. Often these introduce even more options, like match rounds that make your gun more accurate. Or SWC rounds that pierce armor and do more damage, but eat away at your gun faster. And there's even more specialized ammo types - like dragon's breath shotgun shells that turn your riot gun into an imprompteau flamethrower, or pulse slugs that give it massive bonus damage versus robots and power armor.
FNV has more options in gameplay and is generally more nuanced, and in an open-world RPG, more options and nuance is always a good thing.