The only games I have physical copies of ATM are my 3DS games, 3 N64 games, and 2 SNES games, but those were all bought within the past year. If I had to guess, I'd probably say Pokemon Diamond. I got it at launch, and played it through 2009, with over 400 hours logged.
I used to trade in games all the time, cause I didn't have money for new games, and at one point in time (From like 2008 to 2010), I had an "addiction" to trading consoles for other ones on craigslist and whatnot. I've had at least 3 Wiis, 2 360s, 2 PSPs, and other consoles that I've traded for. Right now, all I own is a 360, 3DS, and N64. Had a Wii, but I gave it to my mom and little sister for them to play.