Three major observations from someone who has re-watched both Alien and Aliens quite a few times (and has seen the sequels once each... let's not talk about those haha).

First: given these movies are 30-40 years old, it is insane how well they have held up, effects included. The practical effects work on both these movies is stunning.

Second: these two movies work so well together in tandem, and make perfect sense from a plot perspective with the 50 year gap since the Alien ship was first discovered in between. With Alien, Ridley Scott gave us a really tight, terrifying, what the fuck is going style sci-fi horror movie that still works a treat to this day. With Aliens, James Cameron expanded the narrative scope of the universe in a really interesting way, and made an action thriller that in many ways is just as tense and terrifying as the first movie - turns out that whether you're an unarmed crew or a squad of marines, the aliens are going to go through you like butter regardless.

Third: even when judged just by on its own, individual merits, Alien is an excellent movie. It's just seen it's 40th anniversary, and is still without equal in some areas. Set design that recognises cargo freighters are just as likely to be shit holes in space as they are on Earth. The alien chest bursting sequence - no matter how many times I've seen it, it still haunts me. The reveal of the Science Officer being an Android, the creepy way he stares and does nothing as the alien bursts from another crew member's chest, the gruesome and incredible practical effects when the surviving crew beat him to pieces while he continues to partly function...

And I haven't even started on the alien itself yet. Not only is the creature design spectacular, truly terrifying, and amazingly executed, but the way we get to see so little of it (only glimpses of certain parts at a time while it kills its way through the crew) until the whole creature is revealed to Ripley in the final sequence creates so much tension you probably shouldn't watch this movie if you have a heart condition.

There's nothing that hasn't already been said about Sigourney Weaver already, but her performance as Ripley is awesome. It starts off so low key, the frustration of constantly being overruled by male crew members even though she's the only one thinking clearly enough to look beyond the next 10 minutes, culminating in the fact that she is ultimately the only one strong, crafty, and determined enough to kill the alien and survive.

Whether you look at it as a thriller, a sci-fi movie, a horror film (or all of the above), it's a fantastic work when evaluated as part of any of these genres.

Concluding aside: Fox released a 40th anniversary version of the movie, remastered on 4K UHD Blu-ray. It looks spectacular and is easily worth the purchase if you have the right equipment.