I usually tend to find it's the simplest of games like Tetris, pinball games, Worms and classic racers. Ones with no gimmicks; just real solid gameplay and usually a rock, solid challenge.
Personally I think most people on this site who talk about replayable games always end up mentioning ones only about 3 years old, which I think isn't enough time to prove if they have long replay value. Try a game that you're still playing 10-15 years on i.e. I recently pulled out Pro Pinball: The Web and played hours of it for a few days, a game I've owned for about 12 years now.