I used to use the Big Brother Database from Emacs but these days I don't use anything in particular and I'd like to change that.

I try to use this CardDAV implementation by Sabre/dav myself in combination with the Cardbook addon on Thunderbird; it sets up a personal server you can store the data in. it works well. But I often find myself frustrated that I still have most of the contacts in my old Gmail / Android contacts list on my phone.

https://sabre.io/dav/carddav/