I used a very unusual scale which depends on enjoyment.
10 is a special rating for series that truly captured me much more than the rest.
9 is given for series that I generally enjoyed watching, and may or may not have a good replay value.
8 is a default rating for all anime series that I watch, especially since I created the list at a time when I have watched a lot of old anime series that giving them an exact rating would be a bad decision.
7 means that the series left a bad taste in the mouth.
6 and below signifies worse cases that 7. (TTGL is 6 and is dropped at first episode, and Hidan no Aria is the lowest-rated at 5, dropped at the end of the first story arc.)
The rating system is completely based on personal reaction to the series itself, so I'm sure that this rating system might not work on everyone.