Definitely any of the Black Isle RPGs, Baldrs Gate 1 & 2, Planescape, Icewind Dale. Their hand-drawn art styles don't look too shabby by todays standards(as long as you get a mod to up the resolution, so you aren't stuck playing in an 800x600 window). The Baldurs Gate and Planescape games have the best storylines, but I also still like the Icewind Dale games, they seem to be a little faster paced with more action.
I probably bust them out and replay them every 2 or 3 years, they just never get old. Still the closest thing we have to a true D&D pen&paper experience on a PC