He also dies in the process and sorta loses any ability to directly defeat Vader at that point, so...
He's not exactly the weakest, either. In the timeframe when the Jedi are nearly extinct and the Sith rule the galaxy?
...yeah, he kinda is at the top. Better than any PC Force-user could be, anyway. The only canon Force-users that could be considered more powerful than him after the Clone Wars would be Yoda (not a PC) and the Emperor (also not a PC). Obi-wan would be an edge case.
What if Obi-Wan has the high ground, though? He could beat Darth Vader no problem then, the only reason he loses in A New Hope is because they're fighting on a level surface.
He's also much older than Vader. Also keep in mind, Vader was not defeated by the high ground at any other point in the OT. When Luke got above him, he just threw his lightsaber instead of trying anything stupid. He learned his lesson.