gas.gas: As single player games :
Try Fallout 3, i bought it couple of years ago at a low price as i was curious, honestly i really liked the old fallout rpgs and didnt expect a great game moving from isometric to 3D fps style, instead i really liked it, even unmodded its very solid, fun and if you like to explore it could require more then 200 hours to visit every zone (note then the dlcs are not as good as the game iteslef, especially a couple are just shooters).
FO:NV is a lot better than FO3 in that respect, but FO3 isn't really that bad. As long as the OP doesn't care about it not being a real Fallout game.
The DLCs unfortunately, really varied in that regards. I loved the Pitt and OA, but Mothership Zeta and The Point weren't worth it. I'm somewhat on the fence about BoS, I can't really remember it enough to really have an opinion right now.
But, the rewards, especially from OA, are way too powerful.