I tend to rarely finish something I would rate below a 60. I follow the very simple rule of if I am not enjoying it, why bother finishing it.
As far as ratings go:
91-100 - My top 10, my favorites. 91-100 are my all time top 10's ordered by one point each. There will only ever be 10 scores above 90 on this list, and that will be these top 10. If I love a whole series, such as Mushishi, then the first one will be on this list. If any sequels to it are in the 90's, then I highly recommend the whole series.
90 - The rest of my favorites. At one point or another, they were probably in my top 10.