Sequels in great series don't have that newness factor that makes first films extra special, thus it if difficult for sequels. The novelty factor of Star Wars vs Empire is not to be underestimated, when it came out. Sure nowadays when you can download them sequentially and bingewatch over a weekend, Empire is a better film, but at the time, nothing (not even Empire) beat Star Wars within that genre.
Here in the US the obvious answer is Road Warrior vs Mad Max, since no one saw Mad Max.
As to Alien(s), that is a special case since they are in two different genres. They are both excellent within their genres but fewer patrons prefer the two genres equally so it is difficult to find an unbiased reviewer.