Well, to me a game is succesfull when I play it more than once, or stays more than one year on my system, or through system changes. I don't work so I have a loooooooot of time to use my computer.
Some of the games that comes(or never really made it out) to my memory are
Multiplayer: Ouake3:Arena (TeamArena) many, many hours

(from the day I bought it, two or three weeks after the release)
SinglePlayer:
Quake
Doom/Doom II (also on a Lan with friends) more than two years
Heretic/Heretic II a year or so
Hexen/Hexen II 3 to 5 months
StoneKeep - 4 Times (The story was very good)
Lands Of Lore II: Guardians Of Destiny - 3 Times (Interesting Story)
Lands Of Lore III - Twice (Interesting Story)
Wizards & Warriors - somewhere around 10 days (8 to 12 hours a day)
The Dig - 3 Times
Phantasmagoria - Twice
Pharaoh/Cleopatra - Twice
Settlers II - Twice over a period of a month or so
Dune/Dune II - 3 Times
I am camping in most of the games and some times save/restore a bit to check some of the game-aspects so double the time that the developer defines as playing time.
Mortal - A game freak or a psychotic (it makes no difference)