Oh Snap! Another Past-1AM Update
Punisher. This is an old release (1995) by one of the people who helped make the second Final Doom mission WAD, The Plutonia Experiment. Like Plutonia map32 (and Spark's map01) it's a modification of Doom II map01. It is jam-packed to the rafters with monsters of all sizes (over 600, which in 1995 was ridiculous) Personally I can't finish it without god mode but people with more ability than me can beat it. Its extra constructions and retexturing are quite messy compared to the work done with Plutonia 32, which I prefer (but also cannot beat, at least, I haven't done so yet) Still, Punisher has the advantage of being free to download.
Following a review of a map like Punisher it's interesting to consider the progression of difficulty of maps since the game was made. The typical population of a map from one of Doom's original three episodes is fewer than 50. By the time Doom 2 came out it had broken 100 and when the majority of maps were made in 1995/1996 monster counts ranged mainly from 50 (small) to 400 (very large) Furthermore, the area of a high population map would be correspondingly large, so the density would still be similar. Difficulty is really more about population density than population size.
Of course there were oddities, like Punisher, with its 600 monsters in an area of "Doom 2 map01 plus a couple of large side areas". Now in 1997, Hell Revealed was released. Supposedly a megawad for players of all skill levels, it was mainly noted for its incredible difficulty, and monster density, especially in later levels, many of which exceeded the extremes of Punisher.
After Hell Revealed came the release of the game source code and the removal of certain game engine limits; furthermore, computers were getting faster and more powerful and larger and larger maps could be made. A sequel to Hell Revealed was begun. This project has not yet been completed but a beta version was released, containing some of the most extreme maps ever made. Its map32 was the worst, having something over 1600 monsters in an area not much greater than Doom 2 map 01.
Hell Revealed 2, along with other projects such as Alien Vendetta, and coupled with the power of contemporary hardware, has resulted in the acceptance of obscenely overpopulated maps. Indeed many megawad releases, especially those made by the most skilled players, will now feature one or more extreme maps. The Community Chest Project map29 has over 1300 monsters and the final third of Scythe is almost all extreme in one way or another. The current state of play is that maps with monster count up to 1000 just don't cause that much of a stir any more.
So what does cause a stir? Well, the Nuts series did. Arguably, a lot of the stuff on the idgames archive shouldn't ever have been released, being things like half a dozen square rooms each each containing 40 cyberdemons. Released in late 2000, the original Nuts.wad would have been such a release, but that it pushed the boundary so far that it rapidly became one of the most downloaded maps ever and a useful benchmarking test for source port authors; it had over 10000 monsters, a whole order of magnitude greater than anything seen previously. Recently, we've seen Nuts2, which was a modified Nuts with fewer monsters and more comedy textures and sprite replacements; and just this week we have Nuts3, which... well, to tell you the truth it caused PRBoom to segfault on my system, and apparently only works in the latest ZDoom betas, but I heard it has over 12000 monsters... Oh snap, indeed.
PS Having said all that, I was playing with sandbox this afternoon and made a map with over 24000 monsters with it. PRBoom managed to run it on my system, but the frame rate slowed down so much it wasn't funny. It will be interesting to see when the first playable map with 100000 monsters on it will be released, and whether or not being able to run such a thing will be a sufficient condition to indicate your system can run Doom 3...