#3: Vader vs. Kanaan, Ezra (Siege of Lothal). Lord Vader as an awesome juggernaut vs. two people who are hilariously outmatched, and don't realize it until partway through. (see: #1)
#2: Obi-wan vs. Jango (Kamino). Sure, only one character has a lightsaber, but it's still a s solid duel, and lends credibility to Qui-gon's Episode I line about how even Jedi can be killed, as well as showing just why Jango was chosen as the clone template. Technology vs. The Force, and both sides have to use every trick they have just to stay in the fight. Makes Jango's demise more underwhelming and anticlimactic, however.
#1: Vader vs. Luke, round 1 (ESB). Sure, not the most dramatic or impressive duel, but it's our first real taste of Lord Vader's combat style. Going in, Luke thinks he's awesome...and Vader puts him in his place while holding back. Desperation on Luke's part, Vader's brutality (even while pulling punches), and both characters using the environment to gain advantages, all capped off with some great lines and one of the best twists in sci-fi make this my favorite Star Wars duel. Unlike some others, watching this battle never gets old.