Fallout New Vegas
Unless it was modded like a 1994 Corsa on a council estate (i.e. so much it is hard to tell what the original looked like!) then this game looked utter trash the day it was released.
Heavily modded and it looks stunning. Vanilla is unbearable after you have NMC's texture pack and Project Reality installed
I haven't played it modded. I liked the modding on Oblivion though.
When I played Fallout 3 i thought wow the graphics is poo. When Vegas came out in 2010 I enjoyed it but the graphics was dreadful!.
Well I have posted a few screenies in the new Vegas thread but tbh, they don't really do it full justice as it seems to be sunny in most of them lol, and most of the improvements come from weather mods and lighting.
http://forums.overclockers.co.uk/showpost.php?p=21079024&postcount=2265
http://forums.overclockers.co.uk/showpost.php?p=21088261&postcount=2273
http://forums.overclockers.co.uk/showpost.php?p=21148989&postcount=2284
Not sure I would put it in my top 5 most visually impressive games but it certianly looks much better. The modding just feels like the developers couldn't be bothered to make it look OK - **** it just release in on the Oblivion engine and let people upgrade it for free.