Audio dramas of this quality and this longevity are the unicorns of indie audio drama. Here, there are two long running series: The Red Panda Adventures, which follow the activities of a masked vigilante in the vein of The Shadow and his partner. Black Jack Justice is about hard boiled private detectives, Jack and Trixie Dixon.
The Red Panda Adventures has one of the most satisfying audio drama arcs I've encountered. Superheroes usually don't get to age but here they do, living through the events of the Great Depression, World War II and the rebuilding, and experiencing life events like marriage, having children, and growing old. Through it all, the character development is masterful and no detail is left un-revisited. These are characters who live and breathe. Also, they fight themed villains, Nazis, and dinosaurs. It's the best.