Back when I was a wee young lad: Final Fantasy.
Played it recently on an emulator and forgot how painfully slow-paced it is.
I had experimented with darn near every party configuration you can think of, and I tried to take most of them to the finish.
Just played it on GameBoy and they sped things up a bit. You get money and levels a LOT faster. Less wandering around.
You also dont die as easily and you can save anywhere. Gameplay is much faster overall.
Later on it was Final Fantasy 2 (american, SNES version). Same thing but you cant choose your party. While it was faster paced, I replayed it a buttload of times cuz it was so much fun. Also, I wasted lots of time trying to uncover all the secrets. The stupid "Pink Puff/Adamant Armor" secret wasted about 50 hours of my life.
FF3 was the same way because I always wanted to get all my characters every spell. Leveling up those stupid espers takes forever.
FF7 was probably worse than all of them because you needed a crapload of powerful materia to beat the Emerald and Ruby Weapons. Not to mention some very high levels.
Stopped with the consoles at that point.
I LOVED Total Anihilation: Kingdoms and played the campaign a ridiculous number of times. Some of those missions could take a couple days of playing, saving, and starting up again.
Also spent too much time in the Skirmish Room.
Have probably spent more time on Baldurs Gate 2 than any other game. Its like Final Fantasy 3 only the characters are much more diverse and the optional quests make up a majority of the play time.
Even though the quests are the same every time you play, the game feels different because of your main characters class and your party selection.
Age of Mythology recently replaced TA:K as my big time waster.
I dont even know why but its so much easier to get into than most other RTS's. I just cant seem to get tired of it.
Act of War is a great game, but realisitically I dont think I've wasted that much time on it, not like BG2 or AoM.
Spent a reasonable amount of time on UT and UT2003/2004, but not the hundreds of hours a week that some other folks do.
KOTOR 1 and 2 are probably worthy of the list. Again, they are the exact same games each time you play them, and they dont have the diversity of the the Baldurs Gate games, but I still love to go through both of them at least once a year. Sometimes twice.
EDIT:
Forgot about the Might & Magic games and the Elder Scrolls games.
The problem with those is they are a lot more grind than actual accomplishment. Like MMO's only with better quests. They felt like bigger wastes of time than other games. Even if I beat them I didnt get a sense of doing anything special. I was just glad they were over.
Unlike many people I really didnt waste that much time with Diablo/Diablo 2. They were fun but once you beat the first quest there wasnt much fun in Nightmare & Hell. Its the exact same thing with tougher monsters.
I am approaching the point where I have probably spent just as much time with Titan Quest & the expansion.