The final fifteen minutes or so of Return of the Jedi. That is just such an emotional rollercoaster ride. The perfect resolve of the series. Seriously! I've never understood other Star Wars fans and their underappreciation of ROTJ. It's majestic, the greatest of them all. The whole last hour or so is the very best in cinematic history.
I absolutely agree with this, and this is the Star Wars scene I appreciate the most as a (nominal) adult. However, I think a mention needs to go to the Death Star trench run in A New Hope. I remember seeing that for the first time as a kid, it just absolutely blew me away.

In spite of my love for Star Wars, though, my favourite scene in all of cinema is the Mexican stand-off at the end of The Good, The Bad And The Ugly. The camera work is absolutely inspired, and I don't think the tension it invokes has been matched. The music isn't bad either.