I grew up with games like Doom, Duke 3D, Blood, Unreal.

What I really liked and still like about Doom:
I know how to handle each enemy, I know their strengths and weaknesses. There are places which give you shelter, and secrets which make life easier for you.

Mean traps, weird levels.

And Doom still has those lovely shocking moments where you suddenly start crying because you did not expect this arch vile (or what was the revival guy called?) right behind the door you just opened.

Basically the levels were nonsense, but it was so much fun, and there were so many neat ideas realized with an engine which was so very limited.
A very clever secret passage system where things are shown to you and you simply WANT to get there. You have no idea because the entrance to this secret area could be at the other end of the level, so you automatically started "investigating" each single wall by hitting the space key. Every unaligned or too dark/bright texture was there for a purpose. Never had this in another game again.

The only newer games which come close to this experience are the Serious Sam series. To me they were like bringing back some of the good old times right into the present. Awesome. I have to admit that opposed to the Doom series, Sam shows it's real strength in Multiplayer games. For single player it's a bit too much straight forward - Doom has the better and more clever level design.

My second favorite would be Hexen. It took the Doom game into a fantasy setting and added more complex riddles, making the game a little bit like an adventure. I loved the atmosphere.