Like most people on this thread, I'll acknowledge that The Empire Strikes Back is best film in the series. But I have to go with the original Star Wars as my favorite. Sorry folks, but it will always be Star Wars for me. There was no "Episode IV - A New Hope" when I first saw it, so that's how I remember it. Seeing that movie for the first time brought out a reaction in me that I can still recall vividly to this day. A feeling that I haven't experienced since at any movie (and it's not because it was one of my first date movies either).
I think a lot has to do with the fact that I was old enough at the time to really appreciate how revolutionary Star Wars was both visually and aurally. It wasn't just a "that's cool" kind of reaction. For me, it was a "How the fuck did they do that?!" sense of wonder and amazement. It's impossible for younger moviegoers to fully appreciate this feeling because the visual effects bar has been set so high for so long that they can't remember when images like that didn't exist. (This is something that we can also partially thank Lucas for, regardless of what you think of his writing/directing ability.) But when Star Wars came out, the absolute gold standard was still 2001, A Space Odyssey with it's beautiful but slow (and often static) space scenes. In comparison, seeing Star Wars was like being on acid for the first time. It's hard to believe that the year before Star Wars came out, Logan's Run won the best special effects Oscar.
When I talk to younger Star Wars fans about the original film there's never the same gleam in their eyes that I see in my contemporaries. I compare it to growing up in my home state of Hawaii. As much as I love Hawaii and everything it has to offer, I will never be able to appreciate it in the same way that the average tourist experiences on their first visit. It' not that I don't see or appreciate the beauty. It's the sense of discovering it for the very first time that I don't have.