I really quite liked the ending to the Predator arc, even if I had expected a little bit more out of it. Still, Galgo is my favorite of the Fire and Stone characters, and I'd say he's the most developed, or at least the best developed in terms of writing. Elden had some ham-fisted shit shoved into him later on.
I'd go Prometheus > Predator > Aliens > AvP in terms of ranking the four series.