Starwars doesn't usually go on any list as it's a given, so will always be No.1

My favourite films tend to be those that made the biggest impact on me at the time, and no more modern films can ever achieve this impact again.

But for what its worth these films held me in awe when I first watched them, and still do today, in no particular order of preference, mostly sci-fi :)

Alien

Aliens

Bladerunner

The Thing '82

Invasion of the body snatchers '79

Repoman

Taxidriver

Day the Earth Stood Still '51

Terminator

Alien3 Directors Cut

Silence of the Lambs

Raiders of the lost Ark

Jaws

Dirty Harry

Predator

Ghostbusters