Obi-Wan defeated Anakin because he knew him so well and Anakin was still new to the darkside and made more mistakes. Had Anakin been trained as well as Dooku or Maul, Obi-Wan would have bit the big one.
And yes, George Lucas has said that Anakin is the most powerful force user ever, then Sidious takes the title after Anakin's lava bath.
Exar Kun wins no problem. He almost single handedly resulted in the deaths of several hundred Jedi. Palpatine may have praticaly destroyed the Jedi, but that was with the help of massive armies, bounty hunters, and many other Sith and Dark Jedi. I think I read somewhere that Exar Kun killed over a hundred Jedi in one battle. I think that is more than Palpatine could do.