If Ben is stronger than Yoda (I'm about to listen to it now) then it opens up two possibilities.1) Vader is weaker than Ben, Ben could have owned him in that duel and chose not to.
or
2) Vader himself is more powerful than Yoda.
Although Vader could be more powerful than Yoda (we do not know how powerful ANH Yoda is) I say #1. Obi-Wan lost to save Luke. If he hadn't, Luke would have tried to help him and would have gotten killed. If Obi-Wan killed Vader he'd never even get to the Falcon without getting killed by Stormies, and Luke'd die too.
EDIT: GL said "at this point, the strongest good guy". This could be very well referring to the fact that Yoda had not been introduced yet.
Yes "at this point." meaning at the time of ANH Obi-wan's the strongest. Yoda was still alive, just cause he wasn't in the movie doesn't mean anything.
Ben Kenobi is not more powerful than Yoda
He is, GL, the highest source of canon, said so, so he is.
For Vader vs. Kenobi, I would think that they were on par, like in the ROTS fight. Kenobi either couldn't bump off Vader because they were too equal this time, or because Kenobi needed longer to kill him, but didn't exactly have much time when the others are ready to blast out of there.
"Your destiny lies along a different path from mine."
"Escape is not his plan."
Obi-Wan never planned to leave the Death Star. he planned to be "killed" on the Death Star so he could become a Force Ghost. That's why he lost.