Baldur's Gate and Neverwinter Nights series - great adventures, good character development
Civilization series - no need to explain
Disciples series - this got recommended to me by some friend and I would totally like to buy
Discworld - totally, huge Terry Pratchett fan
Diablo - one of my all time favourites
Leisure Suite Larry - allways made me laugh
Monkey Island - love graphics like that and as far as I remember it had nice storyline
At this moment I don't remember anything else except for old Atari games, like Road Race, River Raid or Alley Cat :) I guess we're not going back in time that much...