These are some of the games that had me hooked the most (when they came out, of course)
Pool Of Radiance/Pools of Darkness (or any from that particular Goldbox series)
Ultima 6
Quake
Quake2 & Quake3 (when I was at school and had an ethernet connection )
Half Life
Edit: Oh, I forgot the Final Fantasy games, (the American versions, at least). Those are still some of my favorites.