Fallout, the first one.
Followed by fallout 2 so close it's a tough decision, and fallout new vegas because it has the fallout feel even though it's a different creature to handle.
Fallout 3 was alright, but I found it rather... I dunno, not gripping, I couldn't get into it as much, partly because the weapons feel shit, and break easily, the environment and characters felt kinda dead, like, too dead. And due to the whole shoehorning in of certain things, like Harold, WHY is harold on that coast? I mean, chances are, if the brotherhood of steel had a hard time getting there, harold woulda been killed off far before he got there, dude's a slow, old, ghoul.
Also he was running a town in fallout 2, so, a little explanation as to why he would want to uproot, and walk across a friggin continent, woulda been nice.