Ultimately, my ratings are based on a mix of enjoyment and value. Enjoyment, or how much entertainment I derived from the anime (or manga), is obviously an important factor, otherwise I probably won't get very invested into the series. However, value is a more objective factor that's often overlooked, and I think it's very important as well. Something that strives to make a lasting impact on the audience - usually by way of introducing thought-provoking ideas or having a unique directional style - deserves a higher rating than something that's just "mindless fun", for instance.
I also can't stand people who don't properly use the rating scheme, namely people who rate everything a 7 or above and aren't at all aware that anything below a 6 even exists.
10 = A complete masterpiece. This is what all anime (or manga) should strive to be. The holistic value of the series completely overrides any minor flaws it may contain. 9 = The title is both very entertaining and very impactful. Not only does it have very few flaws, but it also stands out in some extraordinary way. It's very rare I'll award something this highly. 8 = Any anime or manga that receives this rating is something of high quality. It might have some issues or minor plot holes that can't completely be overlooked, but it's certainly memorable in its own right and stands out among the crowd. Definitely something I would recommend. 7 = A title that reaches this rating is enjoyable, and does what it does well. There are a few obvious flaws or it might be too reliant on tropes, but it's still something I'd say is worth watching. 6 = Generally stuff in this category is fun to watch, or holistically gets its point across. However, it does suffer from a number of issues, or otherwise just doesn't "rise above the pack". Still a favorable rating, and probably the one I give out most often. 5 = A 5 doesn't mean something is terrible. However, it does mean it isn't memorable. Watching or reading something of this caliber isn't a terrible way to waste some spare time, but that's exactly what it is - a time passer. 4 = Titles that fall under this category have their moments, but overall are marred by poor directional style. They might have an interesting premise, and maybe contain a few exciting scenes, but most of the time they're probably either trying too hard or are just flat out boring. 3 = Stuff that I'd give a 3 to wasn't worth my time, and it won't be worth yours. It might have some passable surface-level elements (art, music, etc.) but they mean nothing when the actual story is bad. Expect a lot of awkwardness. 2 = Anything that reaches this rating is really cringeworthy. Beyond just being bad, it also stands out in some terrible way. 1 = The lowest of the low. Titles that receive a 1 are pretty much pure wish-fulfillment or are pathetic in some sort of irredeemable way. Stay away at all costs.