One of the best shooters still alive.

Ports:

Odamex - fps uncapped, lag compensation, no tons of settings yet comfortable to play.

GZDOOM - even more modded. A lot of cool projects for that port such as zombies-doomed-my-neighbors-mod . Also there is way to play any mod online. I have somewhere my batch file..

IWADS:

Doom 1 (Ultimate Doom)

Doom 2

Final DOOM: TNT - Evilution

Final DOOM: The Plutonia Experiment

Master Levels for DOOM II

FreeDoom

REKKR 

WADS: 

SIGIL

fistful3 

mkombat2