My #firstRPG was Ghostbusters... and that has pretty much influenced every game I've played since then. I received the game for my 13th birthday in 1991 and instantly fell in love with the whole concept of RPGs. I went on from Ghostbusters to GBI to Battletech to Robotech and eventually to MERP and Warhammer. But the wacky weirdness (and die pool mechanics) of Ghostbusters has always been my first true love.
My copy (and that of Ghostbusters International, the still fun, but "too machined" second edition) both disintegrated from overuse. I managed to score copies on eBay back in the infancy of the online wars for very reasonable fees. They are now carefully stored on the highest of shelves.