Me? XMPP.

Most used? Depends in what circles. Telegram is popular in some. SimpleX and Session in others. XMPP in free software circles. Matrix in others.

Oh, from the same people who want you to install a mandatory app to access half of the internet? I’ll pass

Do you mean you’d be linked to potential crimes of the previous owner? Well yeah there’s a slim chance but then you show authorities a proof of purchase.

Or if you mean your personal privacy, that’s non existent at the moment you connect to a cell network with a registered sim