1 None - No HTTPS Support/Invalid certificate/Broken or vulnerable cipher or protocols (POODLE, SSLv2 etc'.) Cookies not set as 'SECURE'.
2 Poor - Valid certificate, weak or anonymous cipher suits, none standard ciphers. Site serves mixed content. Any certificate issues like SHA1/MD5 signatures, low rated CA's, lack of revocation lists etc.
3 Good - Fully validated cert and chain, including revocation lists, supports only secure cipher suits with forward secrecy. HTTP-HTTPS redirection or HSTS, all cookies are set as 'SECURE'. No mixed content.
There's only one way to ensure clients aren't MitM'ed on first connect: Go to https://hstspreload.appspot.com/ and submit your site to Chrome's HSTS preload list. Firefox slurps Chrome's list from time to time. The lists are shipped with browser updates, so the whole process takes months.
1. None. Not listening on https.
2. Bad. Invalid cert or broken cipher suites.
3. Ok. Valid cert and good cipher suites, but no redirection to https.
4. Good. Http redirects to https.
5. Great. Redirects to https and sets HSTS header.
6. Amazing. In browser HSTS preload lists.
It may make sense to change the criteria as sites improve, but that list seems sane today. I'd also recommend using letter grades (A+, A, B, C, D, F), but that might cause confusion with SSL Labs[1].
1. https://www.ssllabs.com/ssltest/