Gimmick is up there but it's not the rarest by a long shot. Rodland, Snowboard Challenge and Phantom Air Mission take the top spots. Of the three, only Rodland didn't get a US release. Snowboard Challenge is Heavy Shreddin' and Phantom Air Mission is Ultimate Air Combat (pretty sure, anyway).
There's also a bunch of HES and Gluk unlicensed shit that is hard to come by in some cases.