Just what the title says. I know New Vegas is the fan favorite (and it's mine as well), but what do you personally like replaying the most? Is it Fallout 1, with its tight narrative and quick replay value? Is it 2, with its massive expansion on Fallout 1's formula? Is it 3, with the most atmospheric Wasteland to explore? Is it New Vegas, with the best roleplaying and replayability in the series? Is it Fallout 4, with the best gunplay and survival mode? What are your thoughts?
Typically Fallout New Vegas but I think that's because it feels less linear than Fallout 4. I feel like there's more choice throughout the entirety of the game and because of that (and the lack of essential flagged NPCs) it's great for challenge runs.
The modding community is also great even now. It has been a few years since I've modded a Fallout game and I feel like Fallout 4's modding community's overall activity has probably surpassed New Vegas' since then.
Fallout 4 definitely overtook it for a while, but in the last 2 years or so New Vegas modding had had a bit of a Renaissance! Thanks to NVSE getting picked up again since it was abandoned around 2015, new plug-ins like kNVSE have led to animations being able to be created for each weapon individually, not just weapon category! That led to crossbow and bow mods, huge New weapon animation packs, and more. New Vegas Reloaded adds real shadows, The Living Desert adds more levels of choice and consequence, and Tale of Two Wastelands is better than ever.
If you haven't modded New Vegas in a few years, I sincerely recommend coming back! I'd argue it surpassed 4 again, at least in the last couple years, with regards to how major the mods are that are at play.