Alien, GREAT movie, Aliens also a great movie...
ALL of the other ones sort of lost something in my mind... 3 being not as bad as 4 maybe? Then there is the super terrible AVP movie.
Please leave the series alone, it already has more BAD releases than good.
Alien: great "quiet" SciFi/thriller, underappreciated in this era of "loud/talky" scifi.
Aliens: great, tense, claustrophobic, kickass action/thriller, Cameron at the top of his form.
Alien[3+] : have not wasted my time seeing any of the rest.
Alien prequels announced above: considering in the interviewer Mr. Scott says it's the movie(s) he WANTED to make after the first Alien, I think it'll be more than just a money grab, it'll be an attempt to explore a different story in a different universe, kinda like how the Halo sequels told you more about things you got interested in during the first story. I hope that's what it ends up being, anyway, and I hope it excels in the STORYTELLING component, not the action/thriller element. Certainly don't need any more "duplicate the formula, it made lotsa money the first 1 or 2 times" movie sequels. In ANY series. Sorry, Billy Murray...