Usually listen multiple times before rating, rating system quite self-explanatory- just 1-10 with +5 for albums I think are quite a bit better than others with same rating.
100- Basically perfect
90- Amazing
80- Great
70- Good
60- Decent
50- Average
40- Weak
30- Bad
20- Terrible
10- Awful, practically unlistenable
*Disclaimer- not rating based on what is the 'best', just what I like.*