It's a tall order to narrow it down to three films, since it was Spielberg who sparked my love of movies as a kid and later my fascination with filmmaking as a teenager. But, today, and in no particular order:
Jaws - This is a master class in how to take a potential piece of genre entertainment and turn it into an all-time great movie. Whether by accident, necessity or design, every decision that went into this film was right. Chief among these for me, above the truly great photography, performances, dialogue, music, etc., was their refusal (even with all the horrific difficulties) to back down from shooting it on location. The open ocean is a tangible presence in this film, and a huge factor in its effectiveness. Indeed, Spielberg's later blockbusters are all more technical in their execution, and after a few viewings you can spot the joints in their construction (which doesn't make the good ones any less enjoyable, mind you). 'Jaws', though, still feels organic, and it's not surprising it has held up so well in people's memories...it's one of those rare films that seems better every time I see it.
Close Encounters - As a kid, I just LOVED this movie. I had a healthy respect for 'E.T.', but this one blew it away as far as I was concerned. This was the first of Spielberg's really effects-reliant films, but as nobody had quite nailed down all the processes involved, there's a lovely experimental/improvisational quality to the visuals. The combination of this imagery and the sublime music from John Williams gives the finale the feeling of a dream you don't want to wake up from (I remember watching just the third act repeatedly when I was younger). Growing up in a musical family, the notion that music would be the common medium of communication between us and other worlds was both powerful and quite logical to me. Spielberg and Williams both lucked out with each other on this project. And as for the director's present-day regret that the film's protagonist abandons his family, I've never quite seen it that way...his family abandons him.
Empire of the Sun - This is one of those movies that few remember now, but will increase in stature the more people delve into Spielberg's whole filmography (perhaps like what's happened with Hitchcock and 'Vertigo'). Many seemed rather blindsided by 'Schindler's List' in 1993, but this film pointed the way to that one, while both incorporating and subverting the childlike sense of wonder that had defined many Spielberg films prior. It also tells a fascinating story from a little-known theater of WWII, and is loaded with interesting characters who are perfectly cast (this is my favorite John Malkovich role). The heart of the film, however, is the superb performance by a young Christian Bale, in as nuanced and complex a part as any adult actor could manage. If you'd seen this film, it was unsurprising to see him re-emerge as a big star later in life, and his work is also the strongest testament to Spielberg's ability to direct young performers.
In any discussion of Spielberg, though, I'd be remiss if I didn't mention the impact 'Jurassic Park' had on me as a teenager. While I can't quite put it at the top of the list anymore, I have yet to be more astounded or excited by anything I've seen in a movie than the first time a dinosaur appeared on-screen. While the story and characters don't exactly stand up to the likes of 'Jaws' or 'Raiders', the stars of the show are those dinosaurs, and they are triumphs of the threshold where technology catches up with imagination. These days, CGI is so accepted by audiences that I think filmmakers don't hesitate to throw it at them even if it doesn't really look real. Back then, though, they used it exactly where it would work best, made it look great, then filled in the gaps with practical methods. As a result, those dinosaurs were THERE - they moved with real weight and behaved with true personality - and I became endlessly fascinated with how this wonder was accomplished. That fascination spread to the filmmaking process in general, and it's been a part of my daily life ever since.
Edited by Patrick H. - 9/3/09 at 4:13pm
....Radioman, Adam S, and Sam D....respect!
Needless to say it's one of my favorite films of all time and I've pretty much memorized every line and scene from it. The last time I watched it I was struck by how outstanding John Williams' score is and how it really adds another layer of depth to the film. 
