Cloverfield
This movie blew my fucking mind when I first saw it in the cinemas. I remember seeing the teaser trailer in the months before its release and being addicted to knowing every sliver of information about this mysterious movie. If you weren't there, the teaser had no movie title, and showed a glimpse of the party before it gets interrupted by the 'earthquake', and then you see the Statue of Liberty's head rolling down the road. Incredible stuff.
I remember seeing this multiple times in the cinema, and many many times when it came out on DVD. In the days and weeks after its release I was glued to the internet for the theories and easter eggs and secrets that people had found ("something falls into the ocean?" "there's a secret message at the end of the credits spoken backwards??" "WTF is Slusho?!")
In an age where the Found Footage craze has milked the tropes of the subgenre dry, it's easy to forget just how innovative Cloverfield was. I hadn't seen anything like it before. No one had. It didn't invent the Found Footage genre, it didn't invent the Kaiju genre, but seeing a big monster movie presented this way was incredible. It felt so real and I was straining to catch glimpses of the monster as the camera whirled and reeled, and the scary moments were scary, the comic relief was welcomed (although T.J. Miller has aged like milk...yeesh), and when the monster was revealed it was everything I wanted it to be.
I've enjoyed the Cloverfield movies that have come out since, but nothing beats this one. Probably in my top ten favourite movies of all time.