I think I know EVERY nook and cranny of both Heretic and Hexen. Doom 1 & 2 as well. I pretty much wore out Quake 2 and know it start to finish too.
I also liked the more obscure Descent & Descent 2 and I have Descent 3 but I don't know, it's just not the same somehow. 
There's another old one that I liked called System Shock. I don't think it got very popular, but a couple of years ago I got System Shock 2 and it's really nice.
But then along came Doom 3 and then Crysis and so, well . . .
Still, I have tons of fond memories of the oldies.
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