It's difficult to translate a complex opinion into a numeric scale, so don't necessarily take my "scores" as my set-in-stone grade.
There are many exceptions to my scale and how I rate, but generally, it goes like this:
10 - Masterpiece - These are anime that I view as the very best of the best. Not only are they Phenomenal from a critical perspective, they happen to be one of my personal favorites as well. I recommend these anime to everyone, no exceptions.
9 - Outstanding/Fantastic/Phenomenal - These anime are the backbone of what makes up the anime classics. They are exceptional work that deserve a viewing at least once. Critically speaking, the only thing standing between a 9 and 10 on my scale is that 10's are some of my all time favorites, and 9's aren't as much, though they retain the same objective merits. 9's may have some flaws, but these flaws are negligible in the grand scheme of things and don't get thrown in your face.
8 - Great/Amazing/Awesome - These anime, like 9's, are also classics, but I would say to a lesser agree.They are very much enjoyable and well made. 8's are Anime that I recommend to anybody, however, the difference between 8's and 9's is that 8's may not have the same quality, and may not be for everybody. It's natural that some people just dont click with some anime, but I do think 8's will very likely elicit positive reactions. I respect 8's highly, but they may not be some of my favorite anime ever.
7 - Very Good/Impressive - Enjoyable anime that do their job of entertaining the viewer and making sure their time was well spent. It may not be a truly great series, but it was worth watching and entertained you. These anime have flaws that are visible to most people, but the good elements outweigh them and make for a good show. I would recommend these anime, but I don't fully anticipate other people to appreciate it as much.
6 - Good/ Above Average - These anime are fun or entertaining anime that make them stand out from the average run-of-the-mill anime. It may be possible that these anime have generic or lackluster factors, but they are still enjoyable, and have positive qualities that generally outweigh the negative qualities. It may be the case that these anime had potential to be great, but fell flat due to other mitigating factors. I would only recommend these anime to people who have niche interests.
5 - Average/Mediocre/Meh - These anime are the standard for what I consider to be "run-of-the-mill". Mostly, these are anime that aren't necessarily "bad" but offer little of substance to actually stand out and make it noteworthy. It may be the case that these anime have some positive traits, but they are often negated by the negative ones, leaving you with a "passing" anime. They may be entertaining, but I do not recommend them.
4 - Bad/Poor - These are anime that fail to meet the minimum requirements for what I would consider "Passable". These anime have obvious flaws to any intelligent viewer. There may be some redeeming factors, such as good music, a good character, or nice visuals, but these aren't enough to make up for the abundance of flaws it has.
3 - Very Bad/Awful - An anime that does not even deserve a cursory look. At best, a 3/10 anime will have some superficially positive elements, such as good animation, but at its core is a total mess. Any positive that are present are negligible in the grand scheme, and the plethora of flaws make it irredeemable. It is possible that a 3/10 anime may have had some slight potential at something, but is crippled by its awful elements or awful execution.
2 - Appalling/Horrible - Anime that deserve to die. Atrocities that aren't worth the bits that they are burned on. There may be some slight "so-so" element that prevents it from being the very bottom of the barrel, but those elements are within a storm of imperfections that prevent any real narrative or story from emerging.
1 - Abysmal/Horrendous - If I ever give an anime this score, it really, really, REALLY fucked up, because in my mind, rating something a 1 is just as difficult as giving it a 10. These anime have no redeeming qualities whatsoever, and are infuriating or repugnant to watch. These are anime that I consider to be as bad as The Last Airbender (Movie, not the show), or Scary Movie. So bad.
I see mini-series as their own rating category and scale. Because they are so short, I never really consider watching one a waste of time. In essence, I only rate mini-series 4-6, with very little exceptions.
In this case, my scale becomes a "Check, Check Plus, Check Minus" system as such: