Two words
PasswordAuthentication no
(sshd_config)
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sshd_config - don't forget also:
PermitRootLogin no
(probably the more important one)
then you can allow only SPECIFIC users via 'AllowUsers' 'AllowGroups' etc.
further reduces the possibility of guessing BOTH the user name AND password, unless you disable passwords entirely.
I don't favor entirely disabling passwords. that way you can remote in from ANY machine with an ssh client on it, regardless of whether or not you put the appropriate key into the appropriate place, or are on a dynamically assigned IP address, or something similar. then you pick both a cryptic user name AND a hard-to-guess passphrase (not 'correct horse battery staple' but one like it)
anyway, better than "root:god"
edit: just saw after posting, someone else posted right before me about 'PermitRootLogin'. great minds think alike. 'race condition', he won.