1. Luke Skywalker, no question.
2. Blademaster Kas'im. If you read the Darth Bane book, Bane can only defeat him by using the Force. And even then Kas'im blocks it, it's the temple that kills him.
3. Darth Sidious. Ambidexterity in all 7 forms. That's impressive.
4. Corran Horn. Epic duelist.
5. Caedus/Jaina. He wasn't exactly focusing during their only duel, so we can't say for certain who was better.
I think Saba Sebatyne deserves an honorable mention. She might not be a great lightsaber duelist, but she wrecks in melee fights in general.