The original movie "Alien" was released on May 25th 1979. It's hard to describe seeing the original Alien movie on the big screen for the first time. Suffice it to say, minds were blown like Kane's chest at dinner. Colorful analogy aside, how do you describe someone seeing Alien, or Halloween, or even Jaws for the first time? Things this original and groundbreaking just don't come along this often. It's like the first time you see the T-1000's liquid metal morph, you just realize that you are seeing something amazing for the first time.
Let's get to our subject matter though. Alien has been called a "haunted house" movie. Basically that reductionist view is used to describe a movie where a bad or evil entity pops out and takes its victims while scaring the bejeezus out of the audience. I don't find that very accurate, but Alien was so unique that there just wasn't much else to compare it to.
I did make it my mission to rewatch every movie in the Alien saga and rate them one by one. You have to look at it this way though, you know the name Leonardo Da Vinci, but try to name one painting other than the Mona Lisa. Even though his genius is legendary, everything he does is second to his masterwork. The same can be said for the Alien series. You might even find a heretic here and there who appreciate the action movie vibe of Aliens over the original Alien, but it cannot compare to the moment you see a facehugger, chestburster or Xenomorph for the first time.
There are and have been various celebrations of the Alien anniversary for it's 40th anniversary. There are worthy pieces of merch and short film out there, but do yourself a favor....hunt up the original Alien on some service and experience the wonder and horror of the first movie all over again.