Obi-Wan (TPM) vs. Darth Vader (ROTJ)
I know you guys all hate me, but I'm still posting this.
Here are the conditions:
1. Saber
2. Force
3. All Out
Location: Laser Chamber on Naboo
I'm going to have to go with Vader on this one. He maybe a "crippled, half-droid, half-man" as George Lucas calls him, as opposed to Obi-Wan when he was young, but Vader is no push over.
Then again, that doesn't mean the fight is going to be easy. It's definitely going to be harder than Vader's fight against Luke in the Empire Strikes Back.
Here is my logic. After Maul fatally wounded Qui-Gon, Obi-Wan attacked in such fury that even the ruthless Sith Lord was pushed back. Darth Maul did lose to Vader, but he did gain the upper hand against Obi-Wan, and lost through his own overconfidence.
That being said, Vader would defeat Obi-Wan, but it would be an tough battle for him if Obi-Wan used Force Rage.