The original alien got my attention as a child but the movie Aliens really grabbed my attention. I had to watch it over and over. I was content with aliens until Predator came out and I juggled between the two up until Predator 2 came along. The whole trophy scene blew my mind. Ever since I've become a AVP junkie. I've owned every game, every movie and a wide array of comics, art work, scupltures and novels.
Still don't have a favorite though, It just depends on my mood I suppose.