With millions of modern posts on the Doomworld Forums, hundreds of thousands of modern players,
it is no wonder that with Doom 3 in 2005 and even the new Doom Reboot in 2016, people STILL prefer the best selling game series of all time, the classic and
forever notorious DOOM from 1993. It is obviously not because of its stunning graphics, though in its day these were state of the art graphics. I'm quite certain it's
primarily because of the modding capabilities this game allowed from the very start. Sure they never released level editors for their own game on their own,
but countless third party companies, programmers, and hackers, have developed and still develop countless source ports, level editors, extractors, injectors,
and everything else you can imagine to support users more or less designing their entire own game based on the doom engine.
Doom received critical acclaim and was widely praised in the gaming press, broadly considered to be one of the most important and influential titles in gaming history.
This best selling game of all time definitely earned its place in history, which is now part of the First Annual World Video Game Hall of Fame out of six games ranging from Pong to Super Mario Bros to Warcraft.
It is also dirt cheap on Steam, and only runs you around five bucks to get started for either The Ultimate Doom or Doom II. If, however,
you decide instead to try before you buy, you can always take the WAD file out from a Shareware version and just run it with a modern
Source Port to sample it. The only drawback is without buying the full version, you cannot play any of the 50,000+ custom patch WADS built for it.
Source Ports include the now defunct ZDOOM, which has been around for many years and includes Hexen-like scripting and Boom scripting. It also has its own exclusive ZScript language. It also supports Heretic, Hexen, Strife, Chex Quest, Final Doom, and fan-created games like Harmony and Hacx. The user options are nearly limitless, and most of these source ports remove the actual game limitations as well, such as how big or how high a level can be.
Another of the ZDoom family, GZDOOM, supports all of the same things, plus FraggleScript support, and OpenGL. The OpenGL Renderer makes it possible to have Full 3D Floors, Reflective floors, Dynamic lights, brightmaps, glowing flats, custom shaders, Quake II/Half-Life-style skyboxes, High quality rescaling filters for graphics, sprites, and textures, MD2 and MD3 model support, 32-bit colors (No longer just the doom palette) and True freelook.
The DOOMSDAY Engine exists to refresh the technology of this classic game while retaining the core gameplay experience. It also supports models and most of the things GZDoom does. There is a spinoff source port known as RISEN3D that I've found to look pretty nice. It comes with packs for modern music, textures, maps, enemy and character models, and all the other things to modernize the original DOOM.
If you are interested in researching more, start with this List of Doom Source Ports.