Oh boy, there are a huge amount of graphical improvements mods and "best visual improvement mods" is kinda vague so I'm just gonna recommend some of the bigger ones that won't tax your system too much.
NMC's Texture Pack, use the small pack if your system has trouble running it
Better Game Preformance, as the name implies, it'll increase your preformance
Wasteland Flora Overhaul, use dead wasteland if it's impacting your performance
Enchanced Blood Textures, the wasteland is a very bloody place
Book of Water, this retextures armors and creatures, shouldn't impact your performance much
Realistic Wasteland Lighting, this is a lighting mod, I'm not sure how it'll affect performance but you should be fine
Nevada Skies, this is a preformance heavy weather mod while RWL is a lighting mod so pick which one you wanna use (Don't use both). But be aware, the sandstorms might get annoying or atmospheric depending on how you look at it
That mostly covers it for the big mods, I'm gonna assume that you can't run ENBs so I didn't enclude any. Anyway, enjoy that new, pretty wasteland.