Brad Shepard said:
I would say for me it was Legend of Dragoon, you cant find that game anywhere anymore.
Amazon is your friend:
http://www.amazon.com/gp/offer-listing/B00004S9AF/ref=sr_1_olp_1?ie=UTF8&qid=1267028053&sr=8-1&condition=used
On that note, I bought that same game at a yard sale years ago and despite the bad voice acting, terrible cliches and horrifying graphics, I could never see myself selling it. It's just got that certain charm that makes it awesome. Also, it's fun as all hell.
The main one I regret selling (because I don't really sell that many of them, I know I always end up regretting it) was one of the first game boy games I ever owned: Metroid II-Return of Samus. When I first got my gameboy, I had no idea how to play this game, what the hell was going on, or what to do. I think I played it for maybe ten minutes before I tucked it away somewhere and ultimately sold it. Now that I've since played the living hell out of zero mission, super metroid and fusion, I completely wish I had it back to play again.