Your criteria for rating a climate.

A is an excellent climate with only a few minor flaws.

B is a good but not great climate.

C is a meh climate.

D is a bad but not terrible climate.

E is a terrible but livable climate.

F is unlivable climates.

Don’t really have any rules or criteria except the lowest grade I’d give a “pleasant but boring” climate is a D-.

The biggest hard and fast rule I have is that Tundra=F. Another one I have is hyper arid (less than 1”) is an F. I’ll try to come up with a more comprehensive rating scale later tonight.

Vaguely:

A represents a rather excellent climate that has only a few minor issues.
 

• Summer would have a ~22c mean with 27-29c avg highs, and winter would have ~2c mean with 5c-7c avg highs, although it could be slightly milder or colder in some circumstances as long as snow is reasonably likely during the season while extreme cold is fairly uncommon
  
• Transition seasons should ideally be mild as well, around 18/8c in April and October, with summerlike weather still possible at times in May and September.
  
• My ideal precipitation total would be ~1000 mm/40", think anything from ~25"-55" would be acceptable for an A-grade though.
  
• My ideal sun total is also around 2500 hrs, reasonably sunny but still with a good amount of variety to avoid "stagnation"
  
• Annual mean should be >11c and <14c or so

B probably reflects a good climate that has some minor to moderate issues, whether summers are too hot/mild, or winters are too cold/mild, or being too wet or dry. Many pleasant but boring climates would be in the B category as well.

• I'm generally looking for summers not below 20c means, with at least 25-26c highs to be in this category, and not above 25-26c means either or >32-33c avg highs.
  
• For winter, I'm looking for somewhere where accumulating snow occurs at least once or twice a year on average (so generally not above 13/4), and for nothing below 0/-8c or so (-4c means) where extended, subzero F cold becomes a possibility.
  
• Ideally sun hours would not be <1900 hrs or >3100hrs
  
• For precipitation I wouldn't want to have flooding issues and would like to avoid semi-aridity to achieve this grade
  
• Annual mean should be >9c and <16c or so

 B+: Boston-MA (transitions slightly cool), Toulouse, FR (winter too mild), Milan, Italy (summer a bit too hot)

B: New York City (summer nights too warm), Soria, Spain (too dry), Avignon (too warm year-round)

B-: Detroit MI (winters too cold), St Louis MO(summer too hot and long), Freiburg, GE (gloomy and a bit cool in summer), Portland-OR (boring), Okutama, JP (gloomy), Cannes FR (winters too mild), Gangneung, SK (summer monsoon + humidity + night issues) 



C is an average/decent climate, where I probably appreciate a decent portion of the year but find another part  either overwhelming or really burdensome/irritating, or in the case of milder winters, still pleasant but too bland.

• Annual mean should be >6c and <23c or so 

C+: Porto, PO (bit bland), Le Mans, FR (bit moderated and gloomy), Ithaca-NY (winter and spring are chilly), Barcelona, ES (too warm and dry year round), Atlanta-GA (too warm year-round), Tehran, IR (too hot and dry in summer), Lincoln, NE (too continental), México, ME (too monotonous but comfortable),
C: Wuhan (summer is too hot),  Minneapolis-MN (winter is too cold), Krakow (too gloomy and on the chilly side)
C-: Dallas-TX (too hot), Nicosia-CY(too hot), Hamburg (too cool summers), Montréal-QC (poor spring, slightly mild summer, and harsh winter) , Sapporo-JP (too gloomy + snowy and poor spring) 

D is a fairly poor climate, where I would be uncomfortable most of the year, or severely uncomfortable during at least one part, and the rest of the year doesn't seem good enough to fully compensate for that, but where I still would find liveable (at least a D+)
D+: Harbin, PRC (winters too cold), Stockholm, SE (chilly transition seasons and tepid summers), Hanoi, VT (too hot + humid and poor sun), Phoenix, AZ (far too hot, dry, and blindingly sunny), Dublin, IR (dross weather),  Bergen, NO (bad summers and overly damp)
D: Saskatoon, SK  (too frigid winters + weak summers), Baghdad, IQ (brutal desertic heat), Tampere, FI (just too cold year round), Bangkok, TH (perpetually hot and humid)

• Annual mean >2c and <30c 

E would probably represent a climate that could be survivable on some level but that also seems really unpleasant most of the time. I could tolerate a short stint in a climte like this but couldn't reside there long-term; I use this grade instead of D-.
~Perhaps referring to places like St John's Nfld (too cold, especially terrible spring and weak summer), Torshávn DE (way too chilly in summer and sunless), Krasnoyarsk RU (horribly cold), Mecca SA (disgustingly hot and dry), Niamey NI(scorching)


F: I would use this for absolutely atrocious climates that seem horrible:

F+: Fairbanks, AK (horrific winter), Yakutsk (even more horrific winter), Longyearbyen, NO (no >10c weather in summer and harsh winter), Dallol (furnace)
F:  Barrow, AK, Tiksi, RU, Norilsk RU
F-: Vostok and interior Antarctica (a class of its own) 

A = Excellent climates with Mediterranean or semi arid precipitation patterns, mild winters and warm summers. Las Palmas, Casablanca.

B = This rating includes places with excellent temperatures but lose points for precipitation pattern, eg Sydney, Brisbane.

C = Neutral climates such as London, Paris.

D = These are places with a major flaw in the winter, or extreme hot/wet summers. NYC or Mumbai.

E = Few things good about these climates, but not totally unliveable. Places like Lerwick or Oslo.

F = Vostok, Dallol. People don't live here for a reason.

A: Almost perfect, sunny climates, not too dry, not too cold, not too hot
B: Some flaws but still very livable
C: Half good, half bad
D: No thanks
E: Terrible
F: Unlivable hellholes

Wikiboxes with predominant blue colours will never get more than a D and more likely less.
Red/orange predominant likely A-C depending on the rainfall regime. Monsoonal and Med rate highly.
Frost, ice and snow don't rate although record events do not necessarily reduce a rating.

My criteria will be how closely a climate matches my dream climate, which is still being generated in real time.

A+ = perfect climate

A = excellent climate with maybe one minor flaw

A- = excellent climate with a couple of minor flaws

B+ = very good climate with a few minor flaws, or maybe one major flaw

B = Good climate with a few minor flaws or one major flaw

B- = Solid climate but has a fair amount of flaws

C+ = Above average climate but has a good amount of flaws

C = Average climate with an equal amount of flaws and good qualities

C- = Average climate with slightly more flaws than good qualities

D+ = Slightly below average climate that has some good qualities, but overall meh

D = Below average climate with some redeeming qualities

D- = A bad climate with some redeeming qualities

E+ = Bad climate with a few redeeming qualities

E = Bad climate with a redeemable quality or two

E- = Very bad climate that is still livable for me

F+ = Awful climate with a couple of decent qualities

F = Awful climate with one redeemable quality

F- = Irredeemable climate with zero good qualities, awful in every way

I use a formula, calculated in Excel, weighting 10 different factors regarding temperatures, precipitation, and sunshine hours. Scores out of 4.00 are added up and then divided by 13 (coldest month mean, warmest month mean, and annual precip total are multiplied by two, being the three most important factors). (The 4.00 perfect score comes from American academic grading - in which a 4.00 GPA corresponds to getting an A in every course taken - so a climate that gets a B in temperatures and a D in precipitation can balance out to a C overall). In general I consider 2.70 or higher (a B- average) to be a decent climate, 3.00 or higher being notably good - though given the rarity of extremely high grades I'm a bit more lenient in translating these to letter grades; a 3.02 would be an A- instead of a B for example.