What game have you held onto much longer than you would have guessed? For me, it's my Bride of Pinbot. I just love the game. I have had my BoP longer than any other game in my current collection aside from my Twilight Zone, and at least 40 other games have come and gone, while BoP hangs on.
Why? It's just fun. The multiball is a rush, no matter how many times you play it. Getting the billion point shot is pure joy, and the "she's alive!" sequence is just fantastic. Visitors very quickly understand what is going on and they gravitate to the game. At every party I have, there are at least 2-3 people who park on it trying to progress through the faces. I've also set mine up brutally hard, so it's a good challenge for me to be able to practice repeating 2-3 key shots.
I don't have any desire to move the game out, and I'm sure it will keep hanging on as other games rotate in and out around it.
So yeah, what's your game that has survived much longer than you would have expected?