As much as I like Doom, I think I actually prefer Spear of Destiny. It might be lacking in complexity and suffer from pretty monotonous map design in comparison (although it's an improvement over plain Wolfenstein 3D), but to me the weapons feel like they pack more of a wallop compared to the ones in Doom. Even the pistol feels satisfying to use compared to Doom's peashooter. The enemies feel less like bullet sponges and seem to react more when hit. I like the fact that almost all them use hitscan attacks too. One guard sneaking up behind you can mess you up badly or even kill you on the hardest setting if you're not careful. I find Doom's zombie enemies the most fun to kill, so I really like the idea of a whole game where most of the enemies feel similar except generally pose more of a threat.
Unfortunately, I can't go back to playing the game in its original form since I got so used to ECWolf's map feature. That's a seemingly minor improvement that makes a massive difference in playability.
The classic Doom games are my other favorite FPS games though. I usually stick with Chocolate Doom, Crispy Doom, or PRBoom+. I first started with Doom95, since it came with a CD I bought. After realizing that there were other options out there, I ended up missing the launcher. Only fairly recently did I discover that ZDL exists and that it's pretty much what I wanted. Too bad there are no GUI options for monster changing behavior like in Doom95 though. I tend to play on Ultraviolence with Fast Monsters enabled when I'm playing the original Doom or Doom II IWADs.
Unreal Tournament's probably up there too. I waited way too long to finally try that one. My family had a demo on our computer as a kid, but I was never allowed to actually play it. I remember being interested in he character creation feature but don't recall seeing the actual gameplay. It makes me wonder who put that on the computer in the first place since I don't actually remember watching anyone play it. It wasn't until a few years ago that I saw a YouTube video about it and decided to finally give the game a shot. I'm too scared to try it online, but it's fun to kill bots.