The Best Way to Play Classic Doom

All you need to know

1. Install "Doom + Doom 2" on Steam.
2. Download the latest version of GZDoom from https://www.zdoom.org/downloads. It will automatically find the game files from your Steam installation.
3. Launch GZDoom, go into Options -> Display Options, set "Texture Filter mode" to "None (linear mipmap)". (GZDoom defaults to a blurry texture filter setting that makes the game look worse and unlike it does in its original release and the current official Steam release.)
4. For playing custom maps and mods, use DoomRunner to store mod configurations and launch the game with them.
5. For playing multiplayer with custom maps and mods, use Zandronum, a fork of GZDoom with proper multiplayer netcode added (but with less compatibility with newer mods intended for GZDoom).

Details

GZDoom supports full mouselook with proper perspective rendering that doesn't stretch as you look up and down. It also supports many custom maps and mods that the official release alone doesn't.