I'm (still[!]) listening to Bill Simmons as my number one podcast.
S-Town was holy-cow-fantastic but alas it's 7 episodes are over (for me) so I'm not listening to it at the moment.
Radiolab is a definite whenever they have a new one.
Still hoping for a smashing Serial Season 3.
I've started listening to Freakonomics Radio podcast a little but not consistently.
I really liked the Malcolm Gladwell podcast from last summer, Revisionist History.
Others that I listen to, but a lot less, include Men In Blazers and some of The Ringer's various podcasts.