The movies, series, or creature?
If I had to choose between the movies, Alien is a better film, while it was largely just a thriller with some of the best reveal scenes. The vent part is plain awesome where it ends with the creature, I really kind of wan to beat more of Alien Isolation that keeps the same tone, but I get easily scared. Between the creatures, well since I adopted the predator into some of my gaming and internet handle, I think prefer Yautja (predator's species), although I do think the Xenomorph is also cool, and fun to play in some Alien vs Predator games.
For the series in general, I think it is a little spotty. Alien is intense while Aliens is a cool action movie, and the follow up movies are not that great with things like killing cast off-screen and clone thing. Predator is an alright action movie, Predator 2 take it to good adult place and the city setting making it close to the first but a different setting, and Predators was also a pretty nice action movie and setting. Alien vs Predator is a movie I like, especially with how it is Aliens fighting Predators, and the single Predator in the sequel is a badass, although the Alien side gets quite graphic. I kind of also consider Prometheus to be an Alien film, and it is a movie I quite like unlike other people, and I think it tips the Alien series in the favour.
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Through dream I travel, at lantern's call
To consume the flames of a kingdom's fall