Wow, tough choice. I can't say just yet, in one hand, Darth Vader is a given, so I hate to pick him. In the other hand, the rest of them are all very cool... I'd say Obi-Wan possibly or Maul (only because of the best fight scenes in any star wars movie!) but there are a few others I could pick too.
I haven't voted yet, because I can't make up my mind. Obviously, Yoda is a good pick- but he couldn't defeat Palpatine in combat in Episode 3. Neither one of them was getting the upperhand.
Secondly, Mace almost did defeat Palpatine, but only because Anakin intervened did the Dark Side win that battle.
Third, Obi-wan beat the crap out of Anakin at the end of E3, but the tide turns in E4 when Obi is an old man.
Luke could have killed Vader at the end of E6, but he refused.
(geez- this is more confusing than the BCS)
Qui-Gon was defeated by Maul, but Yoda says in E3 that he has learned how to use his "ghost form" and come back to train Obi-Wan. The only other people to have that happen were Anakin, Obi and Yoda. So it makes you wonder if Qui-Gon became more powerful after death.
Gosh. I haven't even touched on Kyle Katarn, who is by far my favorite non-movie Jedi. He single handedly defeated over 15 Dark Jedi Lords through the "Jedi Knight" games, and he killed thousands of sith apprentices during the process.