I think the ones that work best are the ones that know what they are going to talk about. It can be hard to have enough information to talk about for an hour. Sometimes it seems the panels expect to fill some of that time with a Q&A session but that does not always work. I have seen great panels that no one had any questions for the people. The panels that have notes and multiple topics to talk about I think do well to fill the time and cover everything that the panelists want to cover.
Now, the best panels I have been to are the ones that go off topic is some great way. Sometimes it is people in the industry that just tell stories for an hour. They are entertaining and why it might not be what the panel was supposed to be about it works. Of course some of the worst panels I have been to veer off course and never get to the topic either so it is a difficult line to walk.
The ones I enjoy the most are historical ones, panels done by game creators on how they went about making their game, and ones that cover some aspect of gaming that is very unusual or considered slightly taboo.