This is a long post because I'm catching up. In alphabetical order of filenames:
A replacement E3 from 1995ish. I found this in a thread "Any Wads Similar To DTWID?", in particular the comment
> Bak2hell is alright if you clip past the torches at the end of E3M4.
go back to the start and fight the cacos you ran away from like a man
intrigued me. And indeed I have to agree, you should go back and kill the cacodemons at the end. Don't try to kill them at the start, though, otherwise you won't have enough ammo to do the rest of the map, which is rather top-heavy.
The rest of the wad is old school and quite difficult. There is a variety of level types: castles, mansions, towns, a haunted house. It's sometimes inscrutable and I found myself needing to look at it in the map editor. More often I found myself wishing things weren't so cramped or drawn out. The first map would be better if it gave you a weapon sooner, it's like the start of the original E3M1 where you have to pistol cacodemons and imps but there's more of them.
It's worth playing if you like the real old school stuff, but it's a bit on the annoying side sometimes. Still, I don't regret playing it.
A large sprawling castle map running on E2M1. Made by Jim Flynn (Boom, Eternal Doom etc.) who was famous for this sort of size of map apparently. I'm not sure I've played much of his stuff before, I know him more from code in Boom/PrBoom etc.
Some people called this map ugly, they have no taste. But it is hard to solve, you need to find secrets to get in and find the keys. I found it fun once I had learned it. The outdoor courtyard fights are good, better if you found the weapons obviously. Again this is easier said than done. Basically not going to appeal to everyone, especially those who consider levels should be solvable without the use of map editors.
Full E1-E3 replacement from 2013. Each episode starts off quite easy but gets rapidly harder and starts throwing cyberdemons at you earlier than you expect to see them in a Doom1 episode. Some maps especially the final (E3M8) are outright slaughter maps. This is not unexpected given the author Cannonball has several maps in the various Slaughterfest wads. Others have a chronic health and ammunition shortage that requires strategy bordering on exploitation (E2M6). E2M7 is a remake of a 2002ado map which I didn't realise until about halfway through, I just had this constant, intense feeling I had played it before. E3M9 is the usual copy of E3M1 with a twist. You can telefrag, crush, or otherwise kill without weapon use, all the cyberdemons, even the roaming ones at the end you're actually supposed to fight - it just takes patience.
Anyway, just play it, it's worth it. How many Doom1 replacements are there anyway? 2002ado, DTWID, Base Ganymede, Switcheroom which still hasn't come out of beta ... probably another one I'm forgetting or haven't played, but they're far less common than Doom2 replacement megawads, for some reason Doom1 mappers seem to stop at the single episode boundary.
Yet another large, expansive, and smothered in detail, yet in spite of which still looks like Doom, map from Dutch Devil. That was one tortured sentence. Anyway, this one is still a base, but is perhaps less futuristic and more natural, with a theme of bricks, sewers and caves. In fact it kind of reminds me of Jade Earth, at least in appearance. It's much easier, though, both in terms of opposition, and finding the secrets.
Mostly the weapon/monster progression is carefully steady with a few flashpoints where a teleporter goes off. It largely subverts top-heaviness but remains nontrivial throughout - the place where I died the most often was actually right at the end, there is a fierce arachnotron trap. Frustrating when you've carefully played the rest of the level for 30+ minutes already. But I must appreciate it. Arachnotrons don't usually give me trouble, but these are aggressively teleported right next to you at a rather unexpected moment - I mean, I expected the trap, but not the precise timing of it - and you find you can't get out of their line of sight before all your health is burnt away.
One of the oldest known maps. The file timestamp is in April 1994. Given that, its size and difficulty are remarkable and caught my attention. I am not sure 100% kills is even possible. The larger weapons are at the far reaches of the map, protected by more monsters than you have the ammunition to handle. However, speedrunning it is an interesting challenge and I spent a day or two trying to make a demo for it. It turns out most of the nastiest parts are avoidable, you can just run past most of the monsters, and you get an early berserk box, a powerful tool.
The title comes from a large lift which you encounter early on in the map and which has to be traversed at least twice. However, fortunately it is possible to avoid waiting the second time if you step off it before it finishes rising the first time going up. There is also a tricky jump following soon after the first rise which is apparently very carefully set up to be easy if you walk (not run) forward at full speed - autorun is not a helpful thing here! Fortunately modern engines tend to implement the run key as an exclusive-or (so autorun+run turns running off) and mine is no exception. Note the forwardmove "fudge factor" in PrBoom, that makes its recordings of doom2.exe demos distinguishable from vanilla, is enough to break this jump; I ended up taking the code out of rboom, especially since it's been gone from PrBoom-Plus for years and I've never seen anyone complain.
I love the 1995 oldschool look of this kind of map. A chemical processing factory made of several large buildings with grass around them. There's one full of crates (quite sparsely placed, not a maze) and another with an irradiated control room. Watch the fence, it's electrified or something. Once you get through the red door and another pervasively-irradiated control room, it starts getting weirder, with a giant mountain made out of all the different colours of liquid in Doom2. You can't get back up from here if you jump down to it. The map finishes with a not-unexpected ambush - on repeat plays I saved the invulnerability for it (it's otherwise not really needed after all).
The gameplay is distinctly top-heavy, especially on UV where, as the author cackles in the text file, "there are three of everything"!
Three highly advanced ultra-hard maps, by Ribbiks, the author of Stardate 20x6, buoyed by the success of the latter into doing another wad with a different colour. The theme is "under the sea", which it takes further than most, colouring everything in shades of black, grey, and blue; even the keys are changed to light blue, dark blue, and black. The architecture features a plethora of segmented curves which I am sure are intended to be reminiscent of seashells.
Map01 is an introduction, devoid of monsters, showing off the theme and highlighting the recoloured keys. The second is a kind of fortress which has you circle both ways around an inner keep passing through a variety of difficult encounters, to get the two shades of blue keys. The third is the main event, as it were. There is also a secret map31 that is at least in part a remake of Doom2 map01.
There are some clever tricks, for example, you only have to do the inevitable platform jumping sequences on UV; this wad manages to make, not just things, but sectors skill-dependent too, not something one often sees. However the excess of small curved linedefs can be detrimental to player movement, I remember almost getting stuck trying to squeeze down one secret rock passageway. It also likes to stop you from backtracking, ostensibly to prevent immediate retreat from a nasty encounter, but also making exploration more tedious than it should be when one does not know the map already.
The first half of this map is a base / industrial plant*. After the yellow key it becomes a run through a deep cave. The word I'd use to describe it is "precise" - although at first glance it just looks like any random map from 1996/1997 or so, I got the distinct impression it was precisely crafted, with every linedef placed carefully in both reference to its meticulously credited inspirations**, and the author (vdgg)'s considerable technical insight into the workings of the engine. Or maybe I'm just reading too much into it, but there's definitely a considerable degree of technical competence on show if you look under the hood.
One particularly memorable part was the secret plasma gun: it took me over 20 tries to get it, even after I had watched the demo - which the author only uploaded because people were asking how to get the gun - and knew what the intended method was. Frustratingly, you can bump the weapon - but you can't stop repeating the run, because its sector is marked secret so for 100% you actually have to get up there.
* which you start outside of, under a sky, before a very large door. It was eerily reminiscent of Derelict, but only coincidentally, because that was the last new map I had played before downloading this one.
** Even one of my maps, Elixir, from cchest2, is mentioned in the credits; I am somewhat at a loss for the reason. There is maybe one small room early on, dark ASHWALL2 with a lamp casting some light across, that could be it, but it's such a small thing I'm not sure why I get a citation...