A and S tiers are all great. S tier are the ones that are exceptionally good and i would recommend to anyone. The ones in B are "fine" and C are kinda bad, D are terrible. There is no particular order within tiers themselves.

I tried to avoid "Yuri bait" series for the most part but there are some of those that are gay enough to deserve an inclussion.

Adachi to Shimamura as a story is totally S tier but the anime does not adapt enough of the novels, so the anime goes in A despite the source material being S.