There’s some ageless classics. HoMM3 came out 25 years ago and is still pretty much the top of its genre. Freespace2 more or less shut down the spacesim genre 25 years ago, as well…
That’s more or less the placebo effect at work, though. Most people cannot see “faster” than 60FPS; the only actual upside of running higher FPS rate is that you don’t go below 60 in case the game starts to lag for whatever reason. Now, you may be one of the few who actually see perceive changes better than normal, but for the vast majority, it’s more or less just placebo.
It depends if you are new to the game or not. There’s two big categories of mods - those that only perform modernization and quality of life features, and those that rework everything until it looks like Skyrim.
If you are new, I’d recommend not using any of the latter; they can be fun, but it’s good to know New Vegas “as is”, I’d say. Otherwise the selection is so big it’s hard to pick, I’m running like 30 or so (mostly extra weapons, enhanced AI, better crafting, extra sidequests and a player home). But just give an idea on the scope of mods, the settlement building system in FO4 was inspired by mods originally in New Vegas (Real Time Settler and Wasteland Defense), so there’s really a wide scope of things to pick from.
As for the former, there’s some that jump to mind - NVAC (New Vegas Anti-Crash), FNV 4GB memory patcher, stutter remover, nevada skies and/or EVE (essential visual enhacements), and probably a texture pack or two to enhance visuals. Maybe even NVSE, which is a scripting extension mod that other mods can/will need.