First was likely Zelda (Nintendo). I found every heart by actually searching for them without cheats if that gives any indicator at how much time I spent on that game.

Also on Nintendo, Mike Tyson's Punchout (before he become the politically correct Mr. Universe). That game is all memory... and I could start off at the beginning and make it to the end and win the game without ever getting hit. (you had to block in a few places, but I was never hit ;)

Morrowind is high on the list. (followed by Oblivion).

Thief is also there. Have done full playthroughs of all 3 games over a dozen times and played just about all of the fan missions (hundreds of them).

For the past 5 years, I've spent entirely too much time in LOTRO, but am trying to spend less and less there.... Turbine is greatly helping me in that decision :(

Dark Cloud is another... I can't BELIEVE they haven't tried a 3rd game for that series. Way too much exploring with the camera, fishing, weapon leveling...

Honorable mentions are Diablo 2 coop, Age of Empires 2 coop, Unreal Tournament lan play.

The first game I ever played -- Myst -- took me bloody ages to finish it.

Starcraft, all the Warcraft games, both Age of Empires, Quake, Duke Nukem and the entire Command and Conquer series.

The original Civilization, Railroad Tycoon, Master of Orion, and Master of Orion 2 are without a doubt the guiltiest parties of my gaming past. I played them to death and beyond in their heydays.

I still enjoy a good game of MoO2 today, although, since I have so many more games now as well, I only play it every now and again.

A new challenger has arrived just a couple of days ago: Colonization. Never got to play it back then, so I'm still learning the ropes. Just last night past midnight I was thinking to myself that I ought to go to bed. "I just want to plow this field and build that church and I'm done for the night."... Three hours later I'm finally able to manage to drag myself away from my PC. Good times. ;)

Same here with Alpha Centauri. But just because I was completely defeated by the AI. If not, do not sleep that night ...

Neverwinter Nights by a long shot.

My middle-school years were eaten up by Might and Magic 2 pretty severely.

Neverwinter Nights was the only game that ever made me fear I had a serious addiction problem. I haven't played regularly for a couple of years now, but prior to that I was putting in heavy hours on whatever PW had my interest at the time.

Medieval 2 TW, Silent Hunter 3, Morrowind, Gothic 1+2, SimCity 3000, X2 the threat, Mount&Blade (all of them, even the alpha version), Railroad tycoon 2 and let's not even talk about the Civilization series...

I don't regret even a second of it.

The Legend of Dragoon was a massive one for me.

Remember reading about it in Playstation Magazine, watching the making of feature in Underground, playing the demo in Jampack, I wanted it so badly!

Couldn't have been happier when I got it, don't think I slept that weekend lol

Loved everything about it, still listen to the soundtrack from time to time when I need a potent blast of nostalgia.

Definetly Battlefield 1942, 12 hour multiplayer sessions on a single server, with 1 map and game type.

Days were stolen but awesome times were had.

It's Civ. We all know it's Civ. Nobody that played Civ 1 as a child was able to sleep from then on.

I'm with you, "gamer regret" gets old, especially when there's so many studies showing the positive aspects of all kinds of gaming (not just video games).

Morrowind and Diablo 2 by a long shot, followed by Civ 3 and TIE Fighter. And if I had the choice to do it over again I'd sink just as much time into those games- they didn't steal my life, I gladly exchanged all that time for many great experiences.

I can't really say they were "life stealers" but I would say Fallout, Fallout 2, Gothic 2, Gothic 3, Minecraft, call of duty united offensive multi-player, Persona 4, any rpg, and Medal of Honor: Allied Assault Multi-player. I don't regret them and had fun times.

Right now for me is Morrowind, but any sandbox game is like a drug.
On the consoles is the Dynasty Warriors series (Dynasty Warriors, Samurai Warriors, Warriors Orochi) - I have the DUTY to play with all the characters, gather all the weapons, buy all the horses, play every map in every difficult setting, etc, etc, etc...

Elder Scrolls IV: Oblivion

Fun times...