Meta Doom: A mod that combines every Doom game, even the RPG's (Except Eternal, for now) with new; weapons, demons and items. These are not just lazy sprite copy and pastes but completely new designs and AI. This mod also adds an inventory system and the unholy treasures from the alpha build of the game, it has a polished pacing if you play the campaigns of The Ultimate Doom and Doom II Hell on Earth.
Brutal Doom: Every self respecting Doom player knows this mod and whether good or bad, has an opinion about it, so try it out.
Beautiful Doom: A very nice mod that smoothens the animations of everything and if you want you can modify the behavior of monsters and guns, I myself only use it for the animations though.
Golden Souls (Series): An absolutely beautiful game series that are made in the ZDoom engine. It is an unholy combination of Doom and Mario 64, don't you think it's just a lazy sprite work where it changes Imps with Goombas or some shit, no, this mod has a hub world with a huge mansion and magical paintings that you go trough to play the levels, where you can expect to see new and unique enemies along side your classic imps and what not. The gameplay is a mix of platforming, that surprisingly feels great, and classic Doom combat/secret hunting with completely new weapons. If you find the first game underwhelming definitely try the second one before making up your mind, it's leagues better than the first one.
Doom 4 Vanilla: An incredible and polished overhaul mod that changes everything with DOOM (2016) sprites, enemies and weapons. The highlight of this mod is the fact that you can play it on any port you want, even in DosBox. My favorite Doom mod that adds stuff from other Doom games, definitely give it a try.
In terms of WAD's unfortunately I haven't really played fan made WAD's yet but try the add-ons on the Unity Source port of the games that are on Steam, they look really nice.
There is also a mod called Blade of Agony, which is a Wolfenstein mod for Doom but I haven't checked it out yet, but it looks awesome.
HDoom