For ultra purity, there's basically only Chocolate Doom (or DOSbox and the original game). For modern features (but losing the Doom "feel"), (G)Zdoom or Doomsday; for stuff in the middle, Doomretro, Crispy Doom, Prboom+, Eternity.

I'd say Crispy Doom (or Doomretro on Windows) are good balances.