Podcast directories as well as distribution platforms are a lot like search engines. There are a lot of them, but almost every podcast listener uses the top one.
This is why we say, "Google it" when referring to searching for something online.
When it comes to podcasts, most are found via Apple Podcasts, either directly from Apple's Podcasts app on iPhone or via one of the many other apps that uses its data.
This is good and bad. On the good side, you can reach a lot of people by just submitting your podcast to Apple Podcasts. On the other side, should Apple Podcasts have a policy change and kick your podcast out of its system, you're in trouble.
Taking into account the "risk vs. reward" element of submitting your podcast to various podcast directories and distribution platforms, here are the ones I recommend for the majority of podcasters.
This is the big one. If you're in Apple Podcasts, you're almost everywhere, including the majority of top Android and non-Apple iPhone apps which pull from Apple Podcasts' data.
The Blubrry Podcast Directory allows podcasters to get their podcasts exposure via hardware, software, and devices that are often overlooked, including Roku and Samsung SmartTV.
Podcast Distribution Platforms
If your podcast is listed in major podcast databases, such as TuneIn, Apple Podcasts, and Google Podcasts, note that it's already available via Amazon Echo. However, if you have short (under 10 minutes), frequently-released episodes you may want to make your podcast available via Amazon Echo's "Flash Briefing" feature. This is especially powerful for audio content consumed in the morning, such as news, religious devotionals, and anything designed to quickly inform, inspire, or motivate.
Spotify is primarily a music streaming platform, but because of its large user base, there is a huge opportunity for podcasters. It's especially great for podcasts focusing on a younger audience or related to music.
With approximately 140,000,000 users (paid and free), almost anything you release via the platform is going to get some listens.
You can submit your podcast to Spotify here.
Google Podcasts gets you into everything related to Google, including Google Home and Android phones.
You can submit and manage your podcasts in Google Podcasts Manager.
TuneIn is an audio streaming service that delivers live news, radio, and sports. The TuneIn app streams over 100,000 radio stations from around the world, including AM and FM stations.
TuneIn has a directory, but most people know the service via its app. It's worth being listed on TuneIn since even "non-podcast listeners" who like audio content are familiar with this app and will easily be able to listen to your podcast because of this.
Stitcher, like TuneIn, is an audio streaming service that focuses on radio. Its app is popular on Android.
Sign up to submit your podcast via Stitcher's Content Providers page.
RadioPublic is a Public Benefit Corporation, founded in partnership with PRX, with the mission to "help listeners discover, engage with, and reward creators of podcasts." By being in RadioPublic's database, your podcast will be available via its mobile app.
My Recommended Podcast Distribution Strategy
Don't worry about being "everywhere." Instead, focus on where people in your target audience will easily be able to access your podcast.
If you have an older audience that isn't tech-savvy, for example, get your podcast on Amazon Echo or get a custom app for your podcast. If your audience is younger or you have a podcast related to music and the people you're trying to reach are already on a music-streaming service like Spotify, there's no need to make them go somewhere else to access your podcast when they can do it through the Spotify app they're already using.
The "right" podcast distribution is the one that will allow you to get your podcast to the people who will most benefit. Focus on those people and make their experience listening to you great, instead of spending too much time worrying about potentially picking up a few extra listeners from smaller distribution outlets.