Chaingun, easy. It's DPS is low, ammo capacity is low, and to me it's always been an odd choice to give a marine a chaingun over a battle rifle. Maybe because T2 came out a couple of years before Doom was released? IDK The only reason I use it is sniping or a low shotgun ammo situation. It and the pistol need better DPS options, like dual wielding pistols, and a high RoF less accurate SMG and similar RoF but more accurate Carbine as replacements in the 4 slot. But yea, finding a freakin' chaingun and then realizing it's only capable of ever so slightly more damage per second than a pump shotgun, while that shotgun gives you way more damage from a full ammo pool, that's a real letdown moment.
Grime by EduardoAndFriends
5 Boom compatible maps
Made using Duke 3d textures almost entirely, it's used stylistically to create dilapidated and grimed city environments, its difficulty is easy, but with a couple traps that will kill you if you get too cocky. Very DBP-like in terms of quality, creativity, length, and even an exit-less final MAP06.
The map contains some voice acting by the authors for the zombies, I didn't really like them, but it gave the wad some additional Duke 3D character.
Venus Temple by Azuris
1 vanilla map
A newbie's effort at making a map; it's got plenty of small visual errors and it's very easy, but it's fun and has charm; it reminds me of The Unholy Trinity, and MAP31 from TNT with its basic rooms, but with the author's own custom textures that give the rooms architectural purpose.
Just finished Three's a Crowd. What a high quality WAD! This is what I was talking about. Each map tells its own story and a part of the overarching story. Every map is different and not a single slot is wasted on filler (in my opinion). My favourite level would have to be MAP29 Sheer Poison. Not because I think it's the funnest (I hate the constant damage) but because it clearly communicates through the gameplay itself and the map geometry (where you start in a fake warehouse but really you're in hell), that the IoS is done playing around and is really trying to kill you this time. Combined with the music it gives a sense of hopelessness. That death is inevitable and that your struggles are almost over; give up, you'll die anyway, soon you can sleep. This level communicates a sense of sadness due to the inhospitable environment coupled with the cruel deception at the start. The last level, MAP30, is the only IoS fight I've encountered which isn't a let down. The strength of the multi-map build-up of the IoS and the sacrifices of the Doomguys gives gravitas to this enemy (without relying on the spawner!).
In 2016, there was a guy on Doomworld who called himself the Doom Missionary, and he played Doom on a public library's computer, trying to encourage the locals to get more interested in Doom. The thread he made was dunked on by the forum dwellers at the time, and then it was shoved into Post Hell. Now, Post Hell is no more, and Internet Archive contains stuff up to 2015, so I cannot find it, sadly.