1. Zett Jukassa
2. Jango Fett
3. Coleman Trebor
Zett Jukassa takes, as Janus said, seconds. While the Padawan may be able to hold his own against a few clones, he'll go down rather quickly against the Jedi Master. Jango Fett, while deadly, is going to find himself scorched by his own reflected blaster fire. And Trebor, well, he may be a decent duelist, but he too will fall soon into the battle against someone like Obi-Wan.
4. Barris Offee
5. Aayla Secura
Barris, due to her proficiency with Soresu, may take a minute or two more than some others, but she's really not a challenge for a seasoned warrior who''s mastered Soresu. Aayla won't be much of a challenge, either.
6. Kit Fisto
7. (ROTS) Anakin Skywalker and Qui-Gon Jinn
This is where the fun begins. Kit Fisto would be a considerable challenge for Obi-Wan. He may no longer be Kenobi's superior, but he's damn close to his level. Obi-Wan would, after several minutes of dueling, bring down the Nautolan. Although it would be amazingly more difficult than any of his past battles.
I say he actually has a chance at #7. While Qui-Gon was able to break Maul's defenses twice, and Anakin leanded several blows, even a choke, Obi-Wan won't be easy to overcome, even to these aggressive fighters. They might be able to work well together, but if Obi-Wan can hold them off, he might be able to exploit a weakness in his former Master's form. In all honesty, I think he'll be brought down by Anakin. But if he can take out Qui-Gon, I'd say he has a chance.