1. Alien
2. Aliens
3. Alien3 (either cut, ideal somewhere in between)
^These are some of my favourite films, childhood favourites, read the novelisations many times. My Ripley canon. I give the edge to Aliens over Alien 3, just because it has a more definitive cut and is more fully realised.
4. Alien Covenant (one of the most thematically compelling and subtle films in the franchise, exceeded expectations and breathed new life into the franchise)
5. Prometheus (was just great to see a respectable Alien film again)
6. Alien Resurrection (fun spinoff, loved it immensely as a teen, has that Jeunet weirdness, a fun ensemble and nasty late 90s creature feature)
AVP1 is also solid fun for what it is, Requiem is bad