This is where the lightsaber duel truly became what it is today. If Guinness and Prowse's initial fight was the duel's birth, this is where it came of age.
Empire's climactic battle between Luke and Vader is ground-shaking in all the right ways. Having left Dagobah before completing his training with Master Yoda, Luke should be no match for Vader. And physically, or even in the force, he isn't. It's Luke's ingenuity and smarts that continuously keep the fight going, and allow him to outsmart Vader more than once, and even injure him.
The thrill of watching Luke succeed against the monstrous Vader cannot be understated, which is exactly what makes the finale so devastating.
As Luke loses an arm and is told the truth about his father, it's crushing, not just to him as a character, but to us as an audience.
It remains, to this day, a perfect crystallization of exactly what Star Wars is, and how great it can be.