A.L.T, a megawad from a russian Doom clan. Can be weird and cryptic sometimes, but it offers a very... unique experience. I think it's worthwhile even if megawads are really draining and the quality is all over the place.
Action Doom, fairly old total conversion with almost bullshit gameplay that's still enjoyable. Dying in one hit may turn you away from it though. There's a remade version that includes a better system than one-hit death.
Going Down was previously mentioned, but I should mention it here too. Palatable levels with fun gimmicks and a weirdly good soundtrack. You may also enjoy the wacky sense of humor it has.
The Ultimate Torment & Torture is a sort of cinematic Doom experience, has a lot of focus on looks but doesn't sacrifice gameplay all that much. In my opinion it's the only work by Tormentor667 I can tolerate. (note: i may have my rose-tinted glasses on here, might not hold up as well as i think)
Dimension of the Boomed, Quake recreated in the Boom source port, but has a good portion of originality in it too. Relies a bit much on technical wowing of the player with port-specific level design, but that's sort of the idea of the mapsets the creator is planning to make.
Total Chaos, a total conversion in the same vein as 2012-era horror games like Cry of Fear. Not only is it impressively created in the flimsy ass GZDoom engine, but it's also a good horror experience as well.
Hell Ground, the most un-Doom mapset that still uses Doom assets. Its only issue is the small amount of maps, some of which can feel like filler, but it's good the rest of the way. My personal favorite.
Also worth checking out are some of the Doomer Boards Projects, every month or so they release a mapset with a new theme. Noteworthy examples are Auger;Zenith and Zeppelin Armada.