The Plutonia Experiment (PLUTONIA.WAD)

Dario and Milo Casali were already part of TNT's Evilution, sold to id for them to publish as their own. The Casalis sent American McGee a project of theirs and the maps so much impressed the overlords that they contracted them to make another megaWAD, which was to be sold alongside Evilution as the Final Doom package. The rest, as they say, is history. Plutonia has enjoyed a more positive legacy than its sister WAD, being the beneficiary of two unofficial full-replacement sequels and countless imitators. It's thirty-two more maps of Doom II action, but as anyone can tell you, Plutonia is a beast unto itself.

TNT: Evilution (TNT.WAD)

As you likely know, TNT's Evilution began its existence as a freeware megaWAD developed by Team TNT, who had its origins in a certain Doom mailing list. A bunch of people poured their hearts and souls into the mapset, and just as it was supposed to be released, John Romero popped in, asking if id could buy it, to be published as a commercial work. TNT agreed; thus began a rigorous playtesting / polishing session that delayed the release of Evilution until after TNT put out their freeware sequel, the illustrious Icarus: Alien Vanguard. Nowadays you can find Evilution as part of the Final Doom package alongside The Plutonia Experiment (which is kind of a TNT project, in that the Casalis were members of Team TNT).

Requiem (REQUIEM.WAD)

Most of these authors should look familiar to you. Requiem gathered some of the greatest "star" power when it came to megaWAD authors of the time. Some of the names you may recognize from Memento Mori / MM2, and others from STRAIN and Dystopia 3. And, of course, there are a few new faces, as always seems to be the case with these community projects. The show-stealers for this particular project appear to be Iikka Keranen and Anthony "Adelusion" Czerwonka who together might as well have released Dystopia 4 in disguise. Adam Windsor contributes a fair number of maps as well, but all but one of them are short but sweet affairs, singled out by some as filler speedmaps made to push Requiem to release (given an apparently troubled development, not unlike STRAIN and The Talosian Incident).

Memento Mori (MM.WAD)

Memento Mori has the distinction of being one of the first megaWADs released for Doom II, period, seeing its first publication back in 1995. It's also one of the first community megaWADs, at one time racing neck and neck with H2H-Xmas and TNT: Evilution, at least until id bought up the latter for their own nefarious schemes. Memento Mori has its origins with The Innocent Crew - Thomas and Denis Moeller - authors of two episodes for OG Doom and the partial conversion, Obituary. From what I can tell, they helped form the Memento Mori Crew, gathering up 19 other authors, and pushed their megaWAD out in a little more than five months.