Nextcloud is my best pick. Of course, it does much more than calendars, so if you're looking for a calendar service that does only that it may be a bit overblown.
Big pros:
Native WebDAV integration out-of-the-box - which means that you can easily export your calendars through WebDAV URLs in any supported app
Support for importing external calendars
If you also use other Nextcloud products (like mail, contacts or office) the integration betweren them is quite well designed
On Android then I use DAVx5 to synchronize calendars and contacts from Nextcloud with my phone, and Etar as a multi-provider calendar app.
Nextcloud is my best pick. Of course, it does much more than calendars, so if you're looking for a calendar service that does only that it may be a bit overblown.
Big pros:
Native WebDAV integration out-of-the-box - which means that you can easily export your calendars through WebDAV URLs in any supported app
Support for importing external calendars
If you also use other Nextcloud products (like mail, contacts or office) the integration betweren them is quite well designed
On Android then I use DAVx5 to synchronize calendars and contacts from Nextcloud with my phone, and Etar as a multi-provider calendar app.