Stunt Cycle
Edit: Forgot to add the why
I would guess it is mostly nostalgia, but it is also the lack of commitment that the game requires. I finish the game about every 4th try, and it only takes a few minutes. I play it a few times a month usually, and I enjoy it each time. I guess I have only owned this one for about a year and a half, but it is currently the only classic that I haven't thought of selling. I have owned Donkey Kong for 5 years, and it is my all time favorite game, but I am thinking about selling it. I don't have any desire to play long games anymore. I got to the point that I could score close to 300k, and realized I didn't want to play that long or longer. Knowing that I don't want to play long enough to try to finish the game has really limited my desire play.
Tempest is another that I have had for a while, that I may end up selling. It is nice and quick, because I suck at it, but I find myself playing it less. I bought a Pole Position this year because it is a short game that I may be able to become decent at. It is probably my most played classic (only 4 of my games are classics now) at the moment. I end up playing the pins, Hydro Thunder, or Area 51 when I am out there most of the time.
I have owned and sold probably 50 games, almost all classics. Most of them are fun in small doses, but not fun enough to keep in a 10 - 15 game arcade. I can play them all in MAME with the original joysticks, so that probably contributes to me keeping more of the newer games in the garage. I am usually the only one that plays the classics, but everyone enjoys the pins, racing games and gun games.