This is a list of megawads compiled by Acid at Doomworld forums.
Original here:
https://www.doomworld.com/forum/post/1092593
The following is a list of megawads resulting from a search of the idgames database. Obvs., if there's a megawad not in the idgames database, it won't show up in this list. I don't discriminate; if there's a deathmatch megawad here in this list, that's fine. Also, I make no distinction between Doom and Doom 2 megawads. Read the .txt files to know what you're supposed to be loading the file in. Heretic, Hexen and ChexQuest wads are noted in (parenthesis).