Have you ever found many, if any spinoffs that SOMEHOW turn out better than the mainline games?
For me, even though I still haven't beat it, I've been far more compelled to beat Darksiders Genesis (Switch). I don't know, something about it seems more fitting as a Diablo clone than a mish-mash of Zelda/God of War/DMC etc. Not saying Darksiders is bad as that, but something about Genesis makes it feel more natural.
But for what I have beat, yeah it's Fallout: New Vegas.
Not including the fact that it's my first ever Fallout and finding out it's done by some of the original FO devs and not Todd Howard (though I don't hate his takes on Fallout either), but something about New Vegas still stands out to me. When I played 3 later, I felt like there wasn't as much choice as there is in this one. Yeah, it's rough in the current official state (mods of course fix that), but with several more factions compared to the so few that 3 offered, the varied endings, plus a setting that resonates me as I came from a state one of the DLC is set in (first getting the game when I lived in England at the time). I still go back to this one (following Fallout 4) when I'm in the mood (I've played Fallout 3 the least).