In Our Time is simply the best of the best. From the BBC, a very smart interviewer - Melvyn Bragg - pulls together a couple of experts on a topic, and they discuss it in depth. The range of topics is utterly astounding, and it doesn't matter if you care about the subject or not, you will be fascinated.
This is also broadcast on the radio, but the podcast has a couple of extra minutes of material at the end where Melvyn asks his guests, "Ok, what did I miss?" and they just talk excitedly about the topic they are passionate about.
The range of topics is simply unmatched. Here's a small sample from just the last few months:
Tristan and Iseult
Thomas Paine's "Common Sense"
The Battle of Lepanto
P vs NP
etc, etc, etc