The sheer diversity of Audio fiction makes this a lot harder than you'd think, but a few that I think are worth listening and why:
Hitchhikers Guide To The Galaxy: Probably one of the most enduring Audio-first stories in the modern day (that's right - the Books came after the Radio Show!), and still an enduring influence on a lot of audio producers today.
Welcome to Night Vale: Probably among the first breakout hits of the modern Audio fiction revival, I'd certainly recommend listening to the first two seasons at least, to understand the show that has probably lead to more people considering the medium as a vibrant and viable medium for modern stories.
Wolf 359: Another early breakout hit of the modern revival, an example of a modern audio fiction abandoning it's initial format early on and becoming an incredible show because of it, its producers continue to be active and productive as well in the scene, so as influences go, this one's a doozy.
Greater Boston: I'd include this less for it's influence (though I would claim it is influential), but because of how it plays with narration, and brings the narration into the story in a genuinely amazing way.
Arden: A modern example of Audio fiction imitating and parodying other podcast forms, in this case the True Crime genre, and doing it quite well.
Love and Luck and Y2K: Two quite different examples of the modern revival equivalent of epistolary fiction: the "recorded log/voicemails", and examples of stories that are predominantly "Slice of Life" shows, something that's becoming more common but was quite rare in its time.
What's the Frequency?: Considered one of the most... psychedelic shows in the modern revival, again highly influential among creators, and worth listening to for that alone.
Ars Paradoxica, The Far Meridian, Star Tripper!! and Caravan: The Whisperforge collective is notable for the presence of Mischa Stanton, an amazingly prolific sound designer, as well as the sheer number of producers they either directly helped to produce, or who later produced on their own. Seriously, only the Wolf 359 folk have fostered as much talent, and most of that talent have stayed actors, rather than become producers in and of themselves. You'd want to include at least some of these, probably all, as a set.
Those are the first that come to mind (and most were from my time in production almost a decade ago, when I was listening to the most podcasts) but again, the older podcasts have shown their influence, and part of a classical canon is to capture the pieces that you should know too be able to understand the rest of the corpus.