Podcasts aren't like books. They each do different things well, and in the end it's not all about content. I can look at two RPGs and see which is better at stating its goals, playing towards them, and teaching me how to do it.
Podcasts are different, there's so much more to judge. Some of the best podcasts are at their weakest when trying to present actual game content. Some ar more entertaining, some sound better, some teach you how to play better, some make you want to go out and write, and some make you want to grab some dice and go play.
Which of those things is really the most important?
Last time I did this we tried to go for a broad spectum, select completely different podcasts with completely different strengths, and I'm pretty happy with the way it turned out. Still, I can't help but wonder if there isn't a better way.