It's still holds up amazingly well today... and though there are many games I have loved (Fallout3 for example) I just don't go back to them.
I still remember the day I purchased it and spent the whole time walking back from town reading the manual from the enormous cardboard box... those were the days @_@.
I don't tend to hold onto unused systems and games (or any other media, really), so once a year I spend a couple weeks prowling eBay for a good price on a GameCube and a cheap copy of the game, beat it a couple times, and sell them off again.
Had the same memory card since GameCubes were still on store shelves tho, and the Japanese white GameCube controller since around the Wii launch.
Dragon Age: Origins. I always seem to come back and do a replay every year, and every time I play I hear some piece of dialogue or participate in some quest I hadn't before. It's a game I never get sick of, and it truly baffles me that there are people out there who call it a bad game.
Solitair
Huge time sink while browsing & listening to music. Strangely relaxing.
Surely has the most play time of all my games ever.
Highscore on standard: 12905 pts @>50s
Had +13000pts couple times but pc had to be formatted .
Been playing CS since 1.6, briefly, but got really into it for the Source beta in summer 2004. On and off ever since. I just reinstalled GO last night actually...
I come back for each WoW expansion, unwillingly, but that's been since late 2004 so CS is top answer.
Funny that they're both PC games that are extremely relevant today.
Phantasy Star I on the Master System. I've played it start to finish countless times. By now, I have all maps memorized, so I can play the japanese FM sound version on my Sega Saturn, even though I can't read a single word in Japanese.
Now it's Smite. FPS controls and a good graphics style (along with competitive PVP and a lot of different modes) keep me around. I've actually been pretty addicted to it for about 4 months now. But I've played it off and on for over a year.
New Vegas(thank god for mods). Sometimes I used to alternate between my ps3 copy and my pc version, now its just the PC version tho since I gave my ps3 copy to a friend of mine
Donkey Kong Country 2: Still the best game ever made, I play it at least 3-4 times per year. The Legend of Zelda: The best Zelda game. Same as DKC2, I play it at least 3-4 times per year. Super Smash Bros. Melee and Brawl: This is our "party games".
Honorable mention: Rock Band (1,2,3): I stopped playing because I'm unable to calibrate the game properly with my new TV and Surround System