Personally, Aliens is the best one for me. It has an amazing mix of drama, horror and action that has no visible flaws aside from a somewhat weak performance by the Weyland-Yutani guy.

Alien is an excellent film and one of my top 10 horror movies of all time but the whole thing with the cat at the end seemed rather silly to me. I'm not a pet person so you can call me callous but going back for the cat seemed like a really stupid decision by such an intelligent character as Ripley.

I like animals, but nope it isn't a smart decision to risk your life to save a cat.

It's a movie.  I'm pretty sure if there was huge acid aliens that just killed the whole crew, no matter how much you love a pet you aren't going to try and save it.  In Aliens I'm not trying to save Newt at the end either.   Of course humans will probably die out long before we have the technology for deep space traveling where we might have to make that decision. 

Aliens for me but they are both iconic and brilliant

I don't like the idea of answering your question, but lemme go rewatch them both real quick and continue to not answer your question.

I dont value the life of a cat equal to a human being.

And I would risk my life only for a very loved person.

You make a fair point. Ripley being an extremely intelligent and competent individual isn't mutually exclusive with her desire to save the cat. I recognize that now. It's just that, personally, I don't consider the choice to put your life at risk for a pet to be a good idea in any way, shape or form. As Don Ferrari has said, I value the lives of humans a thousand times more than animals. It's just my personal belief and I don't put my values above yours or anyone else's.

I would also like to point out what you said before, Ripley doesn't seem to hold the pet in any high regard during the rest of the movie so that may be a flaw. Though I have rationalized through the many times I've seen the movie that, after losing so many friends and people dear to her, saving the cat was perhaps some sort of personal goal or redemption that Ripley wanted to achieve. Not that I agree with it! lol, but I can at least understand that point of view.

I don't know what is the problem on recognizing that a smart person made a dumb decision. Most movies and other media is based on people making dumb decisions. And actually that is my biggest grief with most of media, they portray someone as a genius but then that person make very lousy decisions.

What makes it interesting for me is her motivation for making that decision. Was it just a personal flaw? Is she really that big of a pet person? Aliens made her appear to be one but what's strange is her lack of interest in the cat during the rest of the movie. Was it a flaw in the story that the writers missed? Or did she have some other reason to risk her neck for the cat like I mentioned before? It's hard to say for sure.

I'm probably over-analyzing this, lol, but it has always fascinated me since it's really the only serious problem I have with the movie and I still really enjoy it regardless.

I find it very hard to choose myself as they both so fantastic in very different ways; Alien for its atmosphere and dread, Aliens for its pacing and excitement.

Beyond those two, I actually think Alien 3, though flawed, is underrated. Everything after that though is bad in my opinion.