Theatrix, 7th Sea, and Elfquest.
I've managed to use most of everything else one or more times.
If I expanded it to games I'd only used once I would simply list Everway - which I got to play once, but have not managed to GM.
If I expanded it to games I'd only been able to do a few times I'd list Albedo along with Everway, as I've run it two, maybe three times - and everytime it's misserably failed to impress the players, but I still see the wonder of it as a system and a setting.
These days I'm very selective about what I buy, making sure I know how I can use it, so the problem of unused games has not hit me since buying 7th Sea and failing to get anyone to try it due to how bad the setting came across (despite the wonderful system - western players are willing to do nearly insultingly bad stereotypes of asia, but not europe).