> five star rating systems suffers from massive score inflation

Indeed.

I liked how Goodreads had their original rating scale done: 3 was good, 4 was very good, and 5 was truly exceptional. You weren't supposed to give 5 to more than maybe 2% of the books you'd read.

Then the got bought out (by Amazon) and the scale got soon enough devalued.

I still think that one potential solution to the scale inflation would be to consider the grade distribution a person uses. If all they give out is ones or fives, their reviews should have near zero weight. If they give out a more balanced (on a ~Gaussian curve) reviews, then their rare extremes should be weighted much higher.