That would be Legend of Zelda Links Awakening, by far my most replayed game ever. Other contenders are Ocarina of Time, Mario 64, tony hawks 2-4, ssb melee, Mario kart series.

tekken tag tournament (ps2)

Used to be Warhawk, but servers are dead.
So now it is TF2... i cant stop!!!

Final Fantasy Tactics.  Seriously, if you said I could only play one game for the rest of my life, I'd pick FFT without a second thought.  The game-play is so addicting and perfect.  It makes me sad beyond belief that Square has never gone back to the formula depite the game probably having sold billions of copies by now.  The challenge of getting all the hidden items out of Deep Dungeon is one of the most frustrating and rewarding things in gaming.  Just writing this makes me want to go complete another perfect save.

More recently:

Borderlands & Borderlands 2.  I've beaten both games many times on many levels of difficulty.  I could probably play through them in my sleep.  General Knoxx's Amory, the DLC from the first game, I ran it so often that I had it down to at science.

Might & Magic: Clash of Heroes.  Just beat it again a couple of days ago.  It has a nice little RPG story but the combat is so fun and inventive.

AudioSurf.  Pretty much run through any new music I get or when I rip an old CD.  Played some Led Zep songs last night.

FIFA

If there's nothing I want to play and I don't decide to do something completely gaming unrelated, I play any FIFA I can find.

Minecraft. I have projects left unfinished that I plan to go back to on 360. Bought it for PS4 and am starting new shit. I might even buy the PC version someday. It is the perfect game to play drunk, or especially high because it is just building and gathering.

Ocarina of Time
Smash Bros Melee
Mario Kart Series
Mario Series especially Mario 64 and Mario 3D World
Halo 2-3
Pokémon Red/Blue
Call of duty 4 and Modern Warfare 2
Rock Band series
Tony Hawk Pro Skater and THUG

any of these games I can pop in and play anytime and it would never get old

runescape and league of legends

I was going to say "nothing", then I saw this post. Love me some Dark Souls. I think I've got nearly 200 hours across 4 characters and have recently been hankering starting up a new one.

Diablo 2. One of the best games in its genre, in my opinion, despite being kind of terrible. I've beat it close to 10 times, and I'll probably start another character soon...

To a lesser degree, Ocarina of Time, Pokemon Red, and Sonic 2006.