Story, Characters, Animation, Soundtrack, and Enjoyment are the categories I consider in ratings.
Art is subjective. It cannot be quantified and broken down by the numbers like math or subject to critical thinking like science. It relies on the way it makes people feel.
Art is more akin to love or faith. You bring to it your own personal experiences, needs, desires, wants etc... and it either speaks to you or not.
9.Amazing(the enjoyment level is huge they are really well done)
8. Very Good(you will certainly have a good time watching it)
7. Average (inside the standard quality that most anime tend to fit)
6.Not good enough(if you have nothing better to watch then you can try them)
5. Bad (not worth watching, unless they are in some way what you are looking for, under 5 i don't bother and i just drop)