I'm with anyone who says BG, Fallout, Diablo and other similar games (i.e. Planescape: Torment, Torchlight and whatnot).
And to add some recommendations I'll name a few old games I like:
Heroes of Might and Magic 1 through 3 (TBS), 3's the best but I like the other two as well.
Might and Magic 6 and 7 (RPG), I haven't played the latter, unfortunately, but it's also really good as far as I know.
Commandos 2: Men of Courage. Good RTS set in WW2. Not your typical RTS game, though.
Day of the Tentacle. Point & click adventure game, my favourite of that type. My experience is a bit limited, but it's a gem none the less. Requires DosBOX or other such thingy to play.
Majesty: The Fantasy Kingdom Sim. Basically what it says, a fantasy kingdom simulator.
Master of Orion 2 (TBS). It's in space. Pew pew with big lasers and stuff. I've also heard that the original is better, but I wouldn't know, not played it (and the second one's awesome enough for me).
Finally, I'm amazed no one mentioned the original Diablo. Play it. It's awesome.