S-Tier Reference quality. The discs people use to demo their setup.
A-Tier Excellent. Clear upgrade over Blu-ray with strong HDR and detail.
B-Tier Solid. Noticeably better than Blu-ray but with some caveats.
C-Tier Disappointing. Marginal improvement or issues that undermine the upgrade.
D-Tier Worse than the Blu-ray. Bad processing or an upscale pretending to be native 4K.
Ratings from bestblurays.com, hidef-digest.com, and avforums.com.