This game needs no introduction. If Wolfenstein 3D opened the door to First-person shooters, and Doom opened the door, then Duke Nukem 3D blew it off of it's hinges. This is the first FPS I remember playing when I was a wee lad where the main character had personality and memorable "one-liners"...not to mention the whole 'giving money to strippers' thing being quite a big deal when this first came out.
If you don't already have this, what are you waiting for?? It's hard to top something like this classic for $6. BUY IT NOW!