Take Fallout 3 if you're really, really into Bethesda Dev's games, by that I mean The Elder Scrolls series. A big, interesting, atmospheric world to explore which never changes am characters are mostly static and badly written.
Fallout: NV is amazing when it comes to the storyline, because it's made by Obsidian Entertainment, whose writing has always made for interesting characters and storylines, more so than even BioWare. However this studio doesn't have experienced QA, so they release things big ridden games, but for the last few years, it's been been patched and mods were made, so NV is perfectly playable.
So in the end I'd suggest Fallout: NV if you value story.Or Fallout 3, if you value world design and exploring over everything else, in which case, just get Skyrim if you don't have it.
Fallout: NV is amazing when it comes to the storyline, because it's made by Obsidian Entertainment, whose writing has always made for interesting characters and storylines, more so than even BioWare. However this studio doesn't have experienced QA, so they release things big ridden games, but for the last few years, it's been been patched and mods were made, so NV is perfectly playable.
So in the end I'd suggest Fallout: NV if you value story.Or Fallout 3, if you value world design and exploring over everything else, in which case, just get Skyrim if you don't have it.