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.

Congestion 1024 (1024.WAD)

Congestion 1024 is a Doom II megaWAD released in 2005, born from the Exquisite Corpse project, in which a collection of mappers created twelve 1024 x 1024 maps and bound them together as a single, contiguous level. I assume either those involved or those observing found the individual puzzle pieces so intriguing that they made an entire WAD out of similarly sized bits, creating 32 levels of intense action. The rules: the player is bound to a 1024 x 1024 area. They are allowed egress as long as they die upon leaving the 1024 confinement. Monsters, of course, are allowed to be outside the area's bounds and frequently are.