what is the fastest and best way to enable public SSH key access to the Linux PC?

This worked for me.

1. Login Linux from which you want to gain password-less, public key access (lets call it client)
2. execute ssh-keygen on that client (to generate public key, using no password for password-less access)
3. execute ssh-copy-id [email protected] -p itsSSHporthere" to send the client's key to the server
4. Now the client's key is in server's ~/.ssh/authorized_keys and client should have password-less access

In case of reverse SSH tunnel, try to repeat the process, only vice versa, client to the server.

---- /etc/ssh/sshd_config for password access ---
password access:
PermitRootLogin yes
PasswordAuthentication yes
UsePAM yes

---- /etc/ssh/sshd_config for only authorized computers ----
(NOTE: you may cut yourself from this computer in case you do not have physical access to it and you have not supplied valid key/s to its ~/.ssh/authorized_keys file):

PermitRootLogin without-password
PasswordAuthentication no
UsePAM no

service ssh reload