Most of my Genesis and Ps1 collection and probably about a 3rd of my Ps2 collection. Pretty much ALL of my Saturn games, but I just started collecting those, so I don't feel too badly about that. I remember I bought Splinter Cell Conviction and had it unopened for about a year and then I ended up trading it in shortly after that without playing it once (I got it for 5 bucks on a sale at Best Buy and got around 20 dollars trade credit at Gamestop)
As far as the game I had longest without playing it... Probably Metal Gear Solid 4 which I got when I first got my Ps3 3 years ago, and still have yet to play it.