If you had to introduce an oldschool Doomer to ZDoom...

[CaptainToenail]

I tried Daedalus, it's a yawn-fest, don't bother

[Phobus]

Too difficult to work out is the main complaint I hear, and indeed, that was my complaint when I played it myself. You invariably get very stuck somewhere, and then give up.

[Remmirath]

I tried to play it. Maps were too cluttered.

[Ceeb]

It was neat for a bit, but I too got stuck and just quit.

It was a sequel to Icarus, wasn't it?

[Phobus]

It was indeed, but as with a lot of ambitious ZDoom projects of the time, it went way beyond the original idea of merely being a sequel to what is largely considered a slightly better megawad than evilution.

[Shadelight]

-ZDCMP#1
-KDiZD
-TUT&T
-Claustrophobia: The Walls Close In (why has nobody mentioned this?!)
-Demon Eclipse
-Psychic
-Super Sonic Doom

[Ryan Cordell]

-Claustrophobia: The Walls Close In (why has nobody mentioned this?!)

Because they're silly pants.

Also, the boss difficulty might have something to do with it.

[scalliano]

Titan
Titan 2 (can't believe I'm the first one to mention these)
TUTnT
Sapphire
Decade
KDiZD
CIF3 (for a laugh)
Project Slipgate
Dissolution

Plenty of game in there to convince anyone

[Lord_Z]

Project Slipgate

That was a good one wasn't it? Unfortunately, I never finished it because Skulltag killed my save file
Serves me right for not using multiple save slots though.

[Orangewaggs]

Weapon mod wise since everyone mentioning levels...
Diaz Last Hours of Purity Hits Collection By WildWeasel
Psychic By Xaser
The Stranger By WildWeasel
Mutiny 2.0 By Woolie Wool
Zero Tolerance By Xaser and Zero Prophet
Dudukrazy's Guns SE, 2, and 3 By Dudukrazy(!? )
Doom 3 Weapons mod(both of them the one by the 2.5 team and Doomero)
GZDoom Advanced By Eliwood(did i spell it right? if not sorry)
Arsenal By Daniel(I just found this too interesting it may have a lot of weapons but some of them are fun to mess with)

Take my opinon for what they are worth

oh btw way for levels

KDIZD
Ultimate TNT
Action Doom 2
Demon Eclipse
Zpack's Various Levels...

[Pinky's ass]

Assault on Tei Tenga