For each of the movies to me, they each did something special that I liked.
Alien was a science-fiction horror, presenting us with a creature that's never been seen before. It took a completely unique design, no longer conforming to the humanoid shape that most alien designs in movies seems to follow. There was nothing human about it, which is why it made such an effective killing machine. It gives characters we care about, and couple of good scares. Aliens took the series to another level, introducing the Marines, a tough group of soldier who fought these creatures (which will be copied by many movies later) with big guns. The movie was an action movie, going in a totally different way than the first. Alien3 returns to the way Alien was, a dark horror sci-fi film, but this time there was a mix of religious themes which made the movie better and darker. Alien Res was the only one I felt that didn't do anything special nor did I like it that much.
Predator was not just a great action movie with tough commandos, and decent characters, but it gave us another unique creature. Though it took on the humanoid form of a typical alien life-form, everything else about it is different. From the design of its face to the purpose it served, it was a creature that was truly amazing. The movie introduces a group of commandos who could take out a whole base by themselves, making them technically the "top of the food chain". And yet here we have a creature who hunts these men like animals, a hunter of man, the real top of the food chain. Predator 2 followed closely the formula of the first movie, but I still like it for its great action.
As for AVP, it was the idea that caught my interest. Two great creatures, each different than each other, pit in combat against each other. Though it was a great idea, and greatly executed on games, it failed in the movie. I find even though the movie was semi-enjoyable, it was still a failure to live up its potential that it could have had.
"There are five possible operations for any army. If you can fight, fight; if you cannot fight, defend; if you cannot defend, flee; if you cannot flee, surrender; if you cannot surrender, die."