Email, probably. Kind of depends on your needs, and how willing other people are to accommodate them. The most secure messaging platform is email with a third party IMAP client using OpenPGP. That way the client and the server are run by different people, and the encryption is based on a verifiable and well known standard. But will other people use that to communicate with you? Probably not. So probably something like Signal would strike a good balance between privacy and ease of use.