Currently - Dragon Age Origins and my time is sinking fast into that game.

I know I'm ineligible to win anything, so I'll keep it short but:

Demon's Souls (PS3). I found it completely absorbing - I didn't notice how many hours I invested in it until I was at around the 60+ mark, but it had breezed by like 10 or 15. I remember 60 hours of Mass Effect feeling like a thousand.

Especially as you become more experienced (jaded?) a gamer, fewer and fewer games come along that can cast that spell that causes you to accidentally play all night. Demon's Souls was one of those games where you say to yourself "Okay, it's 9:30, I'll play one more hour." And then when you finally get to what seems like a good place to stop, you see it's 3:22am. And you have to get up at 7,

Play this game, Anton! I'll make the bold assertion that it's the best PS3 exclusive yet.

Final Fantasy VII. Know how you start the game with Cloud and Barret in the first reactor? Before fighting the scorpion boss, I saved twice. I used one file to actually play the game and I used the other to just play for a few minutes here and there when I was bored or between classes or whatever. And if I ever felt like starting a new game I could use that file and have something of a head start.

So... Like I said... I played a few minutes here and there and before I knew it, I had played that file for THIRTY HOURS. The characters' levels were incredibly high and the lightning materia that Cloud has when you start the game was pretty goddamn powerful. The reactor's boss can't necessarily defend itself against Bolt 2 or 3.

Cakewalk.

Yep, did all of Oblivion, including shivering isles. However my biggest timesuck was in Final Fantasy X, specifically the damn Blitzball subgame. I think I had like 40 hours in just that subgame.

To begin with, I'm a fan of baseball statistics. If you've never heard of Baseball Mogul, its like a fantasy league except you're not playing against real people. And its text based. By all rights, it should be boring as hell. When I first bought it, I sat down to play as the 1980 Montreal Expos at 4 in the afternoon. When I next looked at the clock, it was 4 in the morning and the game year was 2040. My players were drafted, played out their careers, and died. Prospects became stars and flameouts, and then has-beens.

It was so much fun I do it again once or twice a month.

Final Fantasy 2 for SNES. Granted, it didn't have really any optional quests (except for Bahamut really) but I must played that game again and again and again. And then it came out for the DS and I played yet more. I did the same thing with FF3. Actually, if you tallied up all of my Final fantasy time I have probably logged over a month of my life to the FF saga. Maybe more.
2nd Runner up - Resident Evil series.

The worst part about taking a vacation is having to get to the airport at the butt-crack of dawn. I have never been much of a morning person. To ensure that I was up and ready to get to the airport by 3:30 am, my brother-in-law and I stayed up playing the original NES Final Fantasy. We started around 8:00 pm and kept right on playing until we left for the airport. I figured I could sleep on the airplane. Turns out I cannot sleep well on airplanes and my entire first day in Mexico was wasted sleeping in the hotel room!

World of Warcraft. The game is designed as the ultimate time sink. I spent 3 precious years of my life playing that game. The first time I installed it, I played for 24 hours straight. You could spend weeks trying to build up the gold and materials to craft an item and when you finally get it, it becomes obsolete, and you start all over. The game is trully dangerous.

Mafia Wars on Facebook. I don't know why I keep playing. It's not fun. You can't do a whole lot of playing in a setting. It just takes a whole crapload of time. That's all it does. It's endless. The only good thing about it is that my wife no longer thinks I stay up late at night looking at porn. She thinks I'm playing Mafia Wars. And she's right. Well, half-right.

My favorite time-sink game would probably have to be Final Fantasy X. I don't think that one needs much explaining. Instead of progressing in the story line of the game, I first spent hours on end running around in circles battling every enemy I found to gain XP. And it STILL took me hours to beat Seymour.

As a sidenote, I commented before reading the other comments, and just realized how many people have already said Final Fantasy.... Is it possible for me to change my entry? Because my second favorite time sink game would probably be Pokemon Firered (GBA). After beating the elite 4 10 times, I went on a mission to catch em all. Last time I checked my time, it was 103 hours.

New super mario bros. After beating the game twice, I went back and beat bowser jr in worlds 2 and 5 with mini mario so that I could unlock worlds 4 and 7. Then I unlocked all the other side stages and passages. THEN I went and got ALL 3 star coins from EVERY level. Lastly, I spent hours beating my own highscores in the minigames.

I don't know if it's my favorite, but it was definetely my first...

Old school NES Battletoads.

Not the game so much as the one damn speeder bike level!
OMG! I must have been 9 or 10, and all of my free time was spent trying to best that damn level.
I went so far as writing down each obstruction and the direction to move to avoid it.
I started that level as a child and by the time I gave up (I don't think I ever beat it) I had gone through puberty and was in highschool.

Hitting every single gap in Tony Hawk, whatever iteration of the game. The gap list had to be completely checked off or I'd lay awake at night. OCD, much?

Actually, just disqualify me, I already own Borderlands...
And I'll add some more anecdotes:

The clock in Dragon Warrior Monsters for GBC stops at 99:59. Take a guess why I know this. I played for some time afterwards, and I still haven't finished it to this day, though I eventually stopped.

I hit 240 hours on my original Pokemon Blue a couple years ago. Caught them all and then got them all to lv100. I was so embarrassed at how many hours I spent on Pearl that I entered a Gameshark code to put the hours back to 0.

A highschool friend of mine has 1000 hours on CoD4... and 200 already on Modern Warfare 2.

I've never told anyone this, but my time sink game that i've been playing since it came out is super collapse 3 for the Ds. Now you strangers know my dirty shame :(. I have other time sink games, but this one has been going on since 2007 I believe.

Can I nominate?

my mate daz.

452 hours (19 days) on COD4:MW1

Definitely the Final Fantasy Tactics port for the PSP.

I've gotten all my characters to level 99 and I am still working away trying to master every job.

...But the thing about this is, the game only has enough spaces to record a two-digit play time (clocks out at 99:59). So ever since I passed 99 hours, I don't know how much time I've actually sunk into it!

LOZ: Phantom Hourglass. Before I began the game, I promised myslf that I wouldn't use any online guides or walkthroughs. GOD DAMNED TEMPLE!!!!! The first few trips were easy enough, but then the game just became bitch to play. The reward of finally beating the game would have been enough to compensate for it, but then, I learned that even after beating the game, you couldn't save. So the next time I opened the game, I continued from where I had saved last, confused at what was going on. So I tried again. And again.

There were some games i played 100+ hours, like Fallout 3 or Drakensang.

Right now I'm playing Dragon Age which is a great time-sink. At the moment I'm halfways through the game at 60 hours. That's because I collect everything. So when my backpack is full i leave the current location and head back to the next merchant. So it's a lot of running around. Plus I robbed every NPC (about 600) in the game so far.

The original Master of Orion occupied the entire summer of '94. Ouch.

erm... i feel embarrassed to say this but...

My time played on the first character I made in World of Warcraft is over 100 days. That's right. DAYS.

That is only one of my characters too! I have two others that are both over 20 days.