I love them all. But it goes for me:
1. Alien - masterpiece, classic, timeless..
2. Aliens - great sequel, thrilling action sequences, great technical achievement
3. Alien 3 - awesome cinematography, acting, score, mood, Sigourney is brilliant
4. Alien Resurrection - great visuals, off beat style, quirky characters, the only thing is it so different in tone to the other movies.