That was hard to pick. On a purely cinematic, dramtic level, the Luke vs Darth on Death Star wins that one. That was intense.
On a purely choreographic level, Darth Maul battle wins that one. It was fast paced, unique, and just all around damn good.
But, on a "holy crap" level, the Yoda fight wins. The feel of seeing Yoda kick ass for the first time was just awesome. It's lost its luster now, but the first time I saw that I was in shock. Good stuff (but now that it's worn off I'm insanely bored of AOTC)