Lords of Creation seems to be at least uncommon. I talked about it here.
Battle Born came in a magazine 20+ years ago and I talked about it here.
I have Tarantis and some of the old ziploc-baggie format Invincible Overlord stuff from Judges Guild but those are really settings/adventures, not actual games.
I have one of the Cthulu/Melnibone Deities and Demi Gods that is at least a "limited run" but it's just a supplement not a full RPG.
I have a couple of "fantasy heartbreaker" type games that I had never heard of before seeing them in person.
- I have a version of "Phatasy Conclave" that is 3 small paper, uh, "booklets" staple -bound that look like something that would be handed out at a PTA meeting. It's somebody's tweaked version of D&D and is in no way memorable. Heroic Worlds says there was a boxed set for it in 1984 but all I have are the booklets and they don't really look like something you would have put in a box at that time but who knows?
- I have something much more recent, the Peryton Fantasy Roleplaying Game from 2009. It's a 140-odd page softcover that is a pretty direct rip of the D&D SRD that doesn't do anything really special or unique. The classes are a little different, but not enough to matter. It's really just a cut-down compilation of D&D 3E.
In the end despite the large number of game books I own I don't really care about rarity for its own sake. I keep games that I am playing or running, that mean something to me, that I think I might play in the future, that serve as a resource for another game, or that are cool even if I may never get to play them.