Mine is surely the AK-47 mod. I used it in both the games (I think FO 3 also had one) and I just like the weapon for some reason. There was also a mod that made the cities feel more alive by adding more people, but I can't quite recall how it was called.
I really like Nevada Skies. Basically it's a graphical/weather mod, which changes how the sky looks, and gives a lot more weather options. You can have sand storms, thunderstorms, radiation storms, etc. Really makes the game feel more alive.
I haven't tried it but it sounds kind of like the Climates of Tamriel mod for Skyrim. If it's anything like that, then yes I think it adds a lot of immersion.
My favorite mod for NV was Willow. She is extremely well scripted and acts just like a real person. The vanilla companions in the game don`t even come close to the quality that Willow has. Much respect to the developer of this beautiful mod.
This is what I was going to mention as well. Nevada Skies simply gives the game a different outlook which is always nice, especially since it's highly customizable.
I don't remember the name exactly because I played it a few years ago. But the one that improves the survival aspect of the game is great and everyone should try it out. It makes it so it becomes a much more immersive experience.
The AK-47 mod sounds cool. I've never heard of it before! My favourite mods are the Nevada skies and the Bounty Hunting one. That was so fun to play!
