Despite what certain Chinese women with inflated ego and small behind may say, this is not a troll poll. I hold both games dear to my heart and there are reasons for that. Fallout 2 is one of my all-time favourites, and I was looking for a game like that for a decade, no less. Not counting the closest relatives like Arcanum, Underrail and AoD are the only games that scratched exactly that itch and hit right in the bulleye. Comparing the two, Underrail excels in majority of the main components. Storyfag stuff outta the way first: what game has tribals, Chinese martial arts, talking deathclaws, big bad boys in black, Wild West town, porn business and such? There are threads about that on Codex, hell, even MCA admitted that the direction of Fallout 2 is fucked up. Underrail is behind Fallout 1, but obviously more coherent and less batshit crazy than Fallout 2. Combat, what we all like and always need: while Fallout did it first, Underrail did it right. You can't say with a straight face that Fallout combat system is better, because it's obviously not. Fallout is full of useless perks, underused skills and undercooked systems; and while JA2 got its 1.13, ToEE got its Plus version, Fallout 2 never got any major overhaul to ameliorate the shortcomings of extremely short development time. I have seen Jim the Dinosaur thread and fuck him for abandoning his mod.
Of course, Fallout 2 still stands its ground. It has companions, even if you don't need them; more memorable characters, awesome death animations and excellent artstyle, great soundtrack and sound effects, and great quests; and while combat is simplistic it's still fucking enjoyable like nothing else.
It's my own poll, but I can't choose one; so I vote Joe Pesci.