I used to collect games based on the rarity factor (Blaster Duramold, Cosmic Chasm, etc), but I found that the obscure games never got played by anyone, including myself, and collectors who would appreciate the games never stopped by often enough to warrant keeping them. As space became a premium, they tended to be the first to go.
So lately I've been gravitating towards collecting games that I not only find fun, but have some casual appeal as well. For example, my next restoration will be a Centipede; fairly common in collecting circles, but extremely recognizable to the casual player, and if the restoration is done well, still becomes a nice showroom piece.