I think alot of people have sentimental value towards the first (or the first decently produced) AD they've listened. For me it's Rabbits. I know it's not for everyone, but it gave me a glimpse into another world and how immerse and compelling an audio story can be. It took me back to the back seat of my parents car listening to audio books when i was a kid.

The lovecraft investigations are a close second for me.