The Ultimate Doom

This game is literally just Doom 1 playing on an instance of DOSBox, but it’s a classic for a reason. 
You already know the story: demonic invasion, loads of bad guys to shoot, and one doomed space marine. There may be almost twelve different colours in the palette and four notes in the soundtrack, but it holds up in every area where it matters gameplay-wise. Don’t expect out of the box mouselook support or handholding. 
Just get it, install it in three seconds flat, and jump in already.