I like to replay games so there's probably a long list of games I could play a second or third time and enjoy again. It's a part of why I rarely buy games at full price these days, and haven't for a long time now. Perhaps it's an effect of never having many games growing up.
The big three for me are Super Smash Brothers Melee, Mario Kart Wii, and Perfect Dark. These are my three most-played games, in this order, and I'm still not stopping. I played a little MKWii just yesterday in fact, and I had another go at Perfect Dark a few weeks back. I actually haven't played SSBMelee in a little while though... I think part of that has to do with its lack of a widescreen mode (something MKWii and PD both have), it's a little annoying to have to play it either stretched out or with grey borders.
Aside from those, there are other games I have played a bit and can still play today. Mario platformers are in the top slots of that list, along with Zelda games. DKC series, Mario Party series, NES Mega Man series, Tetris series, DooM series. For some individual games there are the likes of Super Metroid, Super Smash Brothers N64, Earthbound, Tetrisphere, Final Fantasy VI, Yoshi's Island. There are probably some more too that I can't think of right now...