Once the sappy warm-ups were over, Sarah Palin hit her stride. She is good when talking about what she did in Alaska. I can see why she has an 80% approval rating. The frequent interruptions for cheering, however, are annoying.
She's not as polished as Joe Biden, but she's much more straightforward in her speech patterns and (to me) she comes off as more genuine.
So as public speakers, I'd rate Obama way ahead of the other three, McCain at the bottom, and Biden and Palin in the middle (with very different styles). In debates, I don't know how they would rank, but I would be surprised if Palin is as good as the other three.