I've watched all four films and I have to say the first two were the best. The first, Alien, was a fantastic (for it's time, 1979)Sci-Fi Horror movie, with a great plot and ending. Yes, it's graphics and special effects are not up to par with the modern day Digital graphics and million dollar explosions that seem to dominate today's movies, but it takes a creative script and equally good acting/directing to make a movie like this work. That's why Alien succeeded and has such a large fan following to this day.
The sequel, Aliens, directed by James Cameron (Terminator I & II, Titanic) continued the saga of Lt. Ripley and the "Alien" in fine style. It is my favorite of the four and has an excellent supporting cast. I'm not going to tell you about it; Just watch it!
So, I've had nothing but good things to say about the first two movies. Why only four stars? Because the third movie really went down compared with the first two. I dont think it's because it was really bad, but more because the first two were sooo good, and my expectations for the third episode were high. It turned out to be slow and a little boring. The first two movies were 5-star material; this one only gets 2.5 stars from me.
Unfortunately, the final movie, Alien Resurrection, fails to improve much on the previous movie. Yes, there's definately more action, but Ripley's clone comes infected with the Butch/Slut gene, and makes her character less likable. The script takes much of the blame here. I dont think she was as comfortable in this role. Winona Ryder should have stayed away from this one. Her character softens the plot and generally weakens the whole movie. I struggle to give it 3 stars.
For my money, I would buy Alien and Aliens, instead of the entire legacy. They are two of the best Sci-Fi movies EVER! The last two, I'm sorry to say, are not worth the extra money.