These are all in my opinion:
1. Anakin and Obi-Wan: This duel was the one I'd been waiting for, for ages. It was just the most epic of all the duels and the most emotional. Two brothers fighting in a deadly environment, trying to defeat each other. Not to mention it was the longest/fastest and best choreographed.
2. Duel of the Fates: This is by far the best part of Episode One. Whenever I watch the movie, I usually fast forward to this fight, just because it's that cool. Very well-choreographed and it was the first 2 on 1 duel; plus it had the double-bladed saber.
3. Luke vs. Vader (ESB): This duel was the most "flashy" of the OT and it was very well done. What made this number three? When Vader cut off Luke's hand and then revealed his true identity, it automatically made it probably the most memorable scenes (or at least one of them) from the OT.
4. Luke vs. Vader (ROTJ): I really like this duel. Father versus Son, for the final time. Vader's redemption. The death of the Emperor. Plus it lead to the unmasking of Darth Vader (my favorite scene out of all the movies).
5. Yoda versus Sidious: Pure evil versus pure good. That's what this duel meant to me. The ultimate bad guy going head to head with the greatest Jedi. Palpy's cackling got old, but I didn't let that ruin the duel for me. I liked seeing them fight, because it was not only a saber fight, but an epic fight of the Force as well.
6. Anakin/Obi-Wan versus Dooku (ROTS): I liked this one beacause it happened near the beginning of the movie, and it was just awesome seeing the space battle go on in the background. It was nice seeing Anakin and Obi-Wan work well this time as a team and as heroes of the Clone Wars. Plus I loved seeing Dooku getting decapitated.
7. Yoda versus Dooku: The main reason I liked this fight: we got to see Yoda fight for the first time, and it was awesome. He was the most acrobatic fighter from the saga, yet the most frail-looking and physically weak. It proved that Yoda was indeed a badass, just in case anyone doubted him.
8. Ben Kenobi versus Darth Vader: To be honest, the choreography of this fight was pretty poor, but that's not what made it so good. The emotions behind it, and Ben's sacrifice made it an important scene/duel.
9. Mace vs. Palps: I didn't care for this duel that much. It wasn't clear enough as to who the "real" victor was and Palpatine uttering UNLIMITED POWER took away from how good it could've been.
10. Obi-Wan versus GG: Average fight. Grevious turned out to be a coward, but I didn't get all excited about him before the film, so I didn't mind. And Kenobi kicked his ass, so it was alright.
11. Anakin and Obi-Wan versus Dooku (AOTC): Another average duel. Anakin was a dumbass and got electrocuted, leaving Kenobi to fight alone against Dooku. Then got his arm lopped off, go figure.
Man, am I tired...