How could it be the "most secure"? How do you as a client verify the self signed certificate is the right one? If someone MITM'ed the certificate and you've never used it before how would you know that the certificate was intercepted? Who do you go to verify you've got the right certificate? That doesn't sound secure at all. It's like asking potential a liar to swear to you they aren't lying and not asking someone else if that person is possibly being dishonest. Sure central certificate authorities have their problems as a concept, but at least that someone else verifying the cert is the real one.