My 6-year-old loves Decoder Ring Theater. I don't think he follows the stories very well, but he loves the voices.
Let me echo the endorse of Decoder Ring Theater, particularly The Red Panda stories. The Black Jack Adventures put my kid to sleep, and we delete those unheard, but she loves loves loves The Red Panda (and, of course, the Flying Squirrel). She also likes the space ones (written by Matt F'n Wallace, iirc) and I can't remember the name of those, but her fav is Red Panda.
I'd also recommend The Radio Adventures of Doctor Floyd.
Those are both fun for adults as well as children.
My daughter also enjoys Storynory and it's bearable for adults but not as much fun as the other two, both of which are done in radio theater mode.
As far as PodCastle goes, my kid's handsdown favorite has been "In The House of the Seven Librarians" by Ellen Klages which she's asked to hear many, multiple times (thank God Rachel's reading of that is so delightful!)