a game that never bores you out even if you completed it

any GTA (self explanitory)

Sly 2 and 3 (my favorite games)

All Ratchet and Clank games (challenge mode proves to be addaicting, in every game. Plus the multipleyer in UYA and co-op in Deadlocked is very fun)

Burnout 3 (best in the series, and first to include takedowns. My favorite racing game)

Team Fortress 2 (this is the best ****based multiplayer experience ever)

Silent Hill 2 and 3 (the scariest games ever created, they never get old)

Gears of War ( I hate to say this, but it's true. This game is just plain fun)

SSX (the first game series that I REALLY feel in love with)

Rock Band (self explanitory)

there are tons more but these are the ones off the top of my head

Multiplayer is too easy of an answer.

Currently, I don't get tired of replaying Ace Combat 6. GRAW, GRAW2 and Vegas 2 have very replayable single player missions. So do the Lego games.

Older games.... Star Wars Battlefront 2.

Everything with great or competitive MP mode. Game like CoD4 or WoW or CS.

RPGs and RTS plus some fighting genres like SF4-Tekken where are competitive games with ranking etc..

In the end depends what genre you like more and for what. You just want to have fun, hardcore gaming or competitive gaming?.. Really depends on many things

STALKER: SHADOW OF CHERNOBYL.

Never gets old.

Metal Gear Solid 2: Sons of Liberty

Zelda Ocarina of Time. I got it 10 years ago when it came out and I've probably beat it over 50 times. Never gets boring.

Lego Racers 2.

Laugh all you want, it's awesome.