[nucleardog]

It Just Works(TM). Easy onboarding, doesn’t complain if you don’t let it vacuum up your entire address book, simple and usable interface, syncing is flawless, native desktop client, first party library for integrations and support of third party clients.

If you’re not willing to sacrifice too much UX for privacy, it kicks the pants off of a lot alternatives while still, ostensibly, not being the worst offender privacy-wise.