Without a doubt - Star Wars. Right from the opening scene, I knew that it's what I wanted to do with my life. It's not my favourite film, but just the work that went into it, all the pain-staking hours spent creating models, sets and costumes, it just made me want to do that all myself!
As for television shows - The X Files was a big one, as was Buffy the Vampire Slayer. The X Files was the first TV show that ever really showed that science fiction could be taken seriously as an art form and drama, and wasn't all about big battles in space.