I've only ever played I.M. Meen (purely ironically, I swear) and Daggerfall on DOS, so I don't really have much input to, well, input.
I would suggest playing Arena and Daggerfall if you like RPGs or Oblivion/Skyrim, despite the latter's mass of glitches. It's apparently still playable, but I've had awful experience, probably from save hacking. Also, the Doom series might be fun if you've yet to play it. I think those are the obvious ones, though.
What have you played thus far?
