For me its left 4 dead 2. i could play that game for hours and never feel like its getting repetitive because of how fun valve made the gameplay and how active the community is.
Minecraft, so many mods that change it up but it all comes down to I just like creating. The only reason i haven't been back in a while is because i don't want to get sucked back in. The temptation is always there for me.
League of Legends. Its basically an addiction at this point, sure i can kick it for a few weeks or months but they always bring me back it with a new champ, or a kick ass game mode which they take out. Damn it Riot we all want URF back, possibly with One for All Hexakill action....think of the 12 Lux Ults every 16 seconds
I go back and re-play my favorites every couple of years: Metal Gear Solid series, Elder Scrolls series and Xenogears mostly. I also play the Souls series a lot too. Also 4X games are hard to get sick of because it's different each time you play: Civ 5, Sword of the Stars, Endless Space, Sins of a Solar Empire.
Pokemon, Half Life 2, Command and Conquer 3, Mercenaries 2, Ace Combat 6 and Minecraft are the biggest ones I can think of.
Pokemon is just always great, Half Life 2 is basically an annual thing for me, C&C3 is a great C&C game and satisfies any RTS need whenever I get bored of others, Mercenaries is my favorite series and game, Ace Combat 6 whenever I get the urge to fly and be awesome and ride into the danger zone, and then Minecraft is just Minecraft. No quests to worry about, no time limits, just me building stuff, tearing the earth asunder with a pick, or wandering around.
FTB Monster takes like 5 minutes to load but it's great. Thaumcraft is a crazy mod that you can get pretty deep into, and there are other mods compatible with it that make it even deeper.
Resident Evil 4
@arbitrarywater: Heroes is definitely a massive engrossing timesink but you can eventually beat it all theoretically; with my 1000 hours or so played across the series I'd say I'm about another 2000 hours away from doing that. You should try the Expeditions: Conquistadors game that was on sale on gog a few days ago; Heroes + Baldur's Gate + Oregon Trail; though it appears to have an emphasis on exploration over combat.
Any Mega Man game, Gears of War 3 multiplayer, Spiderman 3, Bionic Commando 2009. Uncharted 2 + 3, Portal 2, Rotastic, N+, UMvC3, MGS3, Dark Souls, Guitar Hero/Rock Band, Super Meat Boy, Super Mario Galaxy 2, Black Ops Zombies, Infamous 2, any Splinter Cell.
That's just off the top of my head. Those aren't necessarily a list of what I think are the best games of all time but I think it's a pretty great list of games everyone has to play before they die.
I've been addicted to Dota 2 for a year now. I suspect its worse than WoW, which I played for two years, in that there will be no end-game that i'll eventually get fed up with. When I tired with losing, there's the interesting pro scene which just makes me want to play more. Feels a little dangerous.
Nothing. The games that held my attention the longest are probably Jedi Outcast (messing around modding the game or just playing in slow-mo with g_saberrealisticcombat 999), Mount & Blade, and Left 4 Dead 1.
But I always get bored of something eventually. Sometimes I'll go back to it later, like Dota 2 or an MMO, othertimes I'll just move on. I'm not big on replaying single-player games, and I like switching up what I'm playing.