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!).