What first-person shooters do you consider classic?

Well, that's a given. Even the "90s or bust" type folk will make exceptions for games like Operation Body Count, Isle of the Dead, or Witchaven, to name a few.

I consider classic FPS to be Doom, Quake, Duke3D, Half-Life and Half-Life 2. There are plenty of other great titles too but they tend to borrow a lot from the above. Painkiller is one of my personal favourites, and the best slaughterfest type of FPS. It's too bad that everything done with the series aside from the original dev's work is utter horseshite.