Might and Magic VI was drug-like for me back in '98 (or '99?). Anyway, I played it regularly for more than three months.
I've also been addicted to:
Prehistorik 2 (ah, the good ol' times)
Baldur's Gate II
Daggerfall
Might and Magic VII
Ultima VII
Fallout 2
One Must Fall (god, I absolutely LOVED this. I played the demo version like if I was crazy, then I got the full game and wasted another two-three months of my life playing it :w00t
Monkey Island 1 & 2
Indiana Jones and the last crusade/Indiana Jones and Fate of Atlantis
The Quest for Glory series (especially 3 and 4)
and more, but I can't remember.