I've got three main headcanons:
The first is my "if it sucks, ignore it" timeline, where it's basically the movies by themselves (minus the AVPs) plus whatever random bits from the EU fit without too much duct tape. Alien and Predator are separate, and the movies themselves take precedence over other concerns (websites, tie-ins, etc.)
The second is a sort of "broad strokes" continuity, where the general world/tone/whatever is there (ie: there are Aliens, there are Predators, sometimes they fight) but the specific details are sort of vague. There's A Weyland-Yutani Corp, but people can't seem to remember when it was founded. SOMETHING happened on LV-426, but whether Ripley's still alive is up in the air. Kind of place that might be fun to play a tabletop game in, but not really a specific timeline.
Finally, there's this completely insane timeline where as much as possible is crammed into one meta-narrative - the prequels, the comics, AVP, A:CM, the Kenner toys, E V E R Y T H I N G. It is not a healthy thing, this timeline; it's held together with paperclips, duct tape, and a strategically-placed cucumber. It lurches along, subsisting entirely on raw fish heads, making strange gurgling noises as it attempts to portray 'Wilks' and Hicks as two different people. It is repulsive to look at, and must occasionally be restrained for its own safety. I love it all the same.
... Yes, even when it tries to tell me that there were two Runners in Alien 3 and that's why both cuts count NO YOU SHUT UP
