Predator Universe:
Predator
Predator 2
Alien vs. Predator
Aliens vs. Predator: Requiem
Predators
The Predator
[Include all 6 Alien films if you wish]
Aliens vs. Predator (2010)
Most Predator & AVP Dark Horse comics
[Include Aliens novels and Dark Horse comics if you wish]
Predator: Turnabout
Predator: South China Sea
Predator: If It Bleed
The Predator: Hunters and Hunted
The Rage War
The other crossover comics are not canon to the Alien/Predator Universe but there are multiple occasions when it is canon to that particular Dark Horse universe. Judge Dredd is one of these.
Judge Dredd Universe:
Judge Dredd Dark Horse comics
Predator versus Judge Dredd
Judge Dredd versus Aliens: Incubus
Predator vs. Judge Dredd vs. Aliens
Predator Dark Horse comics
There is also the Aliens vs. Predator vs. The Terminator comics whos universe would include:
Predator
Predator 2
Alien
Aliens
Alien 3
Alien Resurrection
The Terminator
Terminator 2
Most if not all Predator, Aliens, AVP and The Terminator Dark Horse comics before 2000
Aliens vs. Predator vs. The Terminator