SPELLJAMMER - F T W
I know, I know.... it was conceptually flawed, mechanically inane, and adolescent at best - but by far, and I mean FAR, it is my favorite TSR/DnD campaign setting. It allows me to connect all of my homebrew with the homebrew of the other DMs I play with - and subsequently with Krynn, Greyhawk, Faerun, Hollow World - the list goes on and on. Of course it helps that my group has gone to great lengths to tweak it and amend it - adding siginificantly more detailed combat capabilities, special maneurvers, tons of new means of propulsion (helms, etc), and taken the simplistic material presented and turned it into detailed, rich and full content. My campaigns, all of them, have a Spelljamming element.
Wow, there are many games I've circled around and played again years after last setting them down (one of the problems with having lots of interests and lots of friends with different game interests and not as much time). I would hate to have to resort to a rule like yours.
I don't think I'd be the kind of person to take other's advice on what to keep in my gaming collection, personally.
As for the question, keep them all. None of them are coming back, most likely, which means you will not be able to get materials for any reasonable price.
I chose to keep Greyhawk, in large because I have every product leading up to the Greyhawk Wars
Incidentally, after weighing my options and the opinions of others, Greyhawk won out - partly because it's the one setting of those three that is easiest for me to find players for, partly because I own tons of it, and partly because of the opinion poll.