Really hard to narrow it down to 5 and especially difficult to separate films that I've seen many, many times because they're great comfort watches, films that I may have only seen once or twice but consider them to be masterpieces, or films that I consider to be excellent for that specific genre or because of a certain time in my life.
If I had to pick 5, I'd go for:
1. Alien - this film essentially ticks all of those boxes above. I watch it at least 3 or 4 times a year and it always feels like I'm watching it for the first time. It's just utterly brilliant in what it sets out to do and the ramp up in tension and suspense is up there with the very best of them. I adore it.
2. E.T. - this is a childhood classic for me, so much so that I have a framed poster of this in one of my rooms. It's magical in so many ways and contains that feeling of what science fiction cinema should be about - that awe and wonder. It's a beautiful film and the ending always gets me, no matter how many times I watch it.
3. The Terminator - the perfect film about the struggle between man vs. machine. It's an excellent thriller and is pulsating at many moments throughout. I remember the first time I watched this and feeling terrified that the terminator just kept coming. James Cameron nailed that feeling of him being an unstoppable machine that would never give up. Great soundtrack, too. Iconic.
4. The Lord of the Rings trilogy - although if I had to choose only one, it would be...The Fellowship of the Ring. I think. Or The Two Towers? They're all perfect and would take up three spots in my top 5. One of my favourite times ever was when a cinema played all three films back-to-back. A group of friends and I watched this and the whole place was packed out. When Sam gave his speech about carrying Frodo in the final film, the whole place gave a cheer. Honestly, I already loved the film to begin with, but that was such a great night.
5. Interstellar - I really struggled to choose between this, Inception, and The Dark Knight trilogy. There's just something about this film that hits the spot with me. Immense soundtrack, great subject matter, and a memorable final section. It's beautiful and one of those films that I think will get better with age.