I have a lot of trouble deciding between Alien and Aliens, but I chose Alien as as my favorite, followed very closely by Aliens. The rest are alright, but drop down a lot on my personal ranking, with Alien 3, then Alien: Resurrection, and lastly Prometheus. I am keeping an open mind in terms of Alien: Covenant and future films, but I don't personally feel that Prometheus did a very good job explaining everything so far. Though I will say that since there are more films coming out (which I wasn't aware of the last time I saw Prometheus), I probably just need to re-watch it to fully appreciate it.