Alien is better, but Aliens is more of a 'fun' movie.

Both of them are badass.. BUT I am going with alien

Both had very different elements to them like Alien being very mysterious and wouldnt show much of the creature until the end but Aliens, much like many other sequels of movies with creatures like Alien and Jeepers Creepers, was in your face just pure action. Plus action is just Camerons style.
To actually answer the question, lol, ALiens is my fave but i wish Jonesy had more of a part of in it.

Aliens has the upper hand but I hate this question.:)

Alien is my favourite but I hate to choose!!