With all the additions to the story since 1977 (particularly Kenobi dispatching Maul like he was cutting through a paper bag), I think it's fair to assume that he probably could have defeated Vader, but instead deliberately sacrificed himself to create enough of a distraction to allow Luke and Leia to escape.
Wait. Didn't Obi-Wan turn his saber and off and just take the hit?
* I just found the fight on Youtube. He very obviously sacrificed himself. Also, Leia's jiggly titties:
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