[deleted]

[TraderEddy]

[deleted]

[DuckReconMajor]

Cyberdreams

[hervoheebo]

Try Scythe and Demonfear.

[Bank]

This one's just a single map, but it's probably one of the first "modern" style maps to come out. Karthik's Congestion Control is my one of my all time favorites.

[Kira]

Revolution!

Edit : Also The Rebirth

[gun_psycho]

If you want something that's not so heavily packed with monsters but still feels like an adventure, try Fragport.

[Pedro VC]

Nuts.wad

[Deeforce]

TraderEddy said:

GREAT wads

There are many great wads [I'm not kidding]! Here you have almost all of them:

Station 1
Station 2
Station 3: Part 1 and Part 2
For some of the wads you need a specific ZDoom version. Get them here or if the link won't exist in the future, then search on Google further on for all ZDoom versions.

[pavera]

Yeah so, another great wad: Mapgame

Not 20+ levels long, but probably my favorite Doom 1 wad ever.

[GreyGhost]

Try the Serenity trilogy (Serenity, Eternity & Infinity) which have 24 shortish maps between them.