Probably Gold. It's my favorite game, and I would have a hard time giving it up permanently in favor of something else. (I would also consider HGSS for the improvements, but there's something about the nostalgia of Gold that I love.)
I would certainly miss the features that the newer games have added, but there's something I find really relaxing about playing the first two generations of games that isn't there in anything newer. I prefer the stat exp system (only have to worry about base stats, then train Pokemon however you want without having to go through a tedious EV training process first), and there's something charming about the graphics. My favorite Pokemon are also from the first two generations, so I would probably be ok on that front. I suspect I would miss modern breeding and such, but if I could only play one game, nostalgia would probably win out.