I consider both Alien and Aliens to be great movies. I see them as almost different genres though. My wife and I went to see Alien before we heard or read anything about it. We figured we were going to see some light sci-fi, probably Star Wars-ish. It scared the living you-know-what out of us!

We loved it!!
Aliens came across more of an action sci-fi. But again...absolutely enthralling!!
I won't talk about the rest of the movies in the franchise, but the first two are on my all time favorites list.