Basic Guide to Source Ports.

This guide will teach you about some of the popular source ports both vanilla and modern as well as how to play multiplayer.

Introduction

I highly recommend finding a source port that isn't the one built into the Steam version. Reason why? Every source port has different features that may appeal to you. If you prefer chunky resolutions and monsters with infinite height or advanced mods with dynamic lights, then the Unity port won't appeal to you much, but maybe one of these will.

Vanilla Ports

If you care about preserving vanilla gameplay/visuals:

Prboom+ (my favorite port), Chocolate Doom, and Crispy Doom.

Note that Chocolate Doom is vanilla only while the other ports mentioned support boom compatibility, basically adding new features like conveyor belts, deep water effects, translucent walls, and many other features. Boom is one of, if not, the most used mapping format, so with Prboom+ and Crispy Doom, you'll be getting a lot of support for community wads.

Modern Ports

If you prefer to play in a non-vanilla fashion (support for mods like Brutal Doom, Russian Overkill, etc):

GZDoom, Zandronum, (note: for Brutal Doom, GZDoom and Zandronum are the only ports that can run it at the moment) Doom Retro (an in between of vanilla and modern Doom)

GZDoom is easily the most popular source port as seen by just about every Steam review suggesting that you use it. It's very easy to set up, user friendly, and allows for pretty much every wad made to be played.

Playing Online

If you want to play Doom Online:

Odamex, Zdaemon, Zandronum.

Odamex and Zdaemon are for the most part vanilla with both being a fork of CSdoom, which itself is a fork of the original ZDoom. Zandronum however is popular for modded Doom multiplayer, being one of two ports that can play Brutal Doom. If you want more information or help setting up multiplayer ports, Doomkid has a video on this you can reference back to.

Conclusion

Of course, these aren't the only ports available. The source code was released in 1998 if I can remember correctly, giving people over 20 years to work on these ports. If you want a more in depth story on the history of ports, Dwars has a video going over the early years of Doom ports.

Also on his channel is a few videos reviewing many other Doom ports and even some Quake ports. I only wrote this guide because I really hate seeing every other source port guide say "use GZDoom" without mentioning any other ports. With a modding history as large as Doom, not mentioning anything but GZDoom would be to undermine so many other people's work.