Looks like I have found my definitive rating system.
Simple: a handful of possible ratings (one to five) that can be assigned without too much thinking and/or calculations.
Absolute: independent from other albums' ratings.

I exclude albums that do not represent a whole/coherent artistic product: therefore no greatest hits, compilations, live collections, non-original soundtracks, etc. A few exceptions allowed (e.g. Marley's Live at the Lyceum).

Many albums have five stars, for two reasons. First, I feel uncomfortable at the idea of giving a score to a work of art, and often I think it's just silly. Maybe this sometimes makes me over-rate albums, but who am I to judge? And who are you to judge my ratings? And who are they to rate your judgment on my ratings? Et cetera. Second, I reject the idea that five stars = perfect album. I don't think perfection in music can be defined. If I enjoy something enough, for a long time, for many good reasons, that's a five stars. Doesn't matter if there's a typo on the back cover notes.

Five stars: Albums that I have been listening a lot, that have at least a few "above the line" songs, with a strong personal meaning in some moment of my life, good and consistent quality throughout. Any of these.
Four stars: Very good albums that for some reason do not make it to five stars. Good, consistent, enjoyable, that I am happy to own. With some specific flaws.
Three stars: Somewhat good albums that are repetitive, with fillers, discontinuous or dishomogeneous.
Two stars: Disappointing, bland, poor albums. Or albums that I just "don't get".
One star: I don't own anything that I think it's this bad. Do I?