Most of the rating schemes are well-established in specific industries

[w=quasi fixed-width]
[ww=fixed-width, great as bullet point]
[x=extensible]

— [ww] smiley 3-levels only .. 🙁 … 😐 … 🙂
more about experience/feedback rather than recommendation/endorsement
— [ww] 👎 👍 thumbs 2-level only
less granular than stars with half-star, but offers negative
— star-rating as in hotels .. Sometimes I prefer star rating with half-stars ★☆
eg: https://btv-open.dreamhosters.com/wp-admin/post.php?post=24863&action=edit&classic-editor

— [ww x] small integer grade.. Sometimes I prefer a numeric rating from 1-5 or 1-10
This is the most flexible, most granular rating scheme
— [x] triple-A as in credit_rating .. Sometimes I prefer S&P + Fitch rating symbols — https://en.wikipedia.org/wiki/Credit_rating

• AAA+ > AAA > AA+ > AA > A+ > A > A—
• The +/- feature is more granular than half-star, but less granular than numeric rating.

— [w x] letter grade as in exams:

• A+, A-, B+, B-
• B is often above-average
• C is often below-average, but in some contexts need to be presented as “decent effort”.
• D and possibly E for more granularity
• F for fail
• The +/- feature is more granular than half-star, but less granular than numeric rating.

— color scheme
deep green
standard green
light yellow
standard yellow
light amber
standard amber
dark amber
light red
standard red
dark red