tokyoghoultheories:

Hi ( ⋂‿⋂’) Rates are based in three things: a ghoul’s strength, how active they are and how dangerous they are to the civilians. There’s six levels: 

• C: 
• Scarecrow: We don’t know how he got his rate, but he seems like he might be stronger.
• B: 
• I don’t remember his name, but the guy Urie fought in Chapter 1 was rate B: He got that rank because he wasn’t that dangerous and was apparently pacific.
• A:
  
• Torso: He got the rate because of how many people he killed in a short period of time; still, he was pretty weak compared to Rize and Tsukiyama.
• S: 
• Rize and Shuu got their rate because they both killed a lot of people and they were pretty strong.
• Nishio: He started hunting and killing ghouls, some of them probably beared high rates; he also managed to deal with the Quinx squad without any problems.
• Jason, Naki, Miza and the Bin Brothers got their rate by being in Aogiri, an organization who kills innocents and ghoul investigators all the time.
• SS:
  
• Shachi and Irimi are both really powerful ghouls who ofter brought trouble to the investigators.
  
• Roma: We saw how powerful and sadistic she is.
• Donato: He used to kill and torture the kids at the orphanage, also giving human flesh to the rest of them.
• Ayato: He’s strong, and he probably keeps going on killing sprees.
• Kaneki: The only one who got his rate just by his strength (and his inestability). Without killing anyone, he defeated multiple ghoul investigators and managed to scratch Arima’s cheek. Even as a ghoul investigator, he remains as a SS rated ghoul.
• SSS
  
• Yoshimura and Eto were thought to be the same person: A really powerful ghoul who killed investigators easily and could only be faced by Arima.

And I think those are all the ghouls whose rate is known.

As for how do they change their rate, Nutcracker was considered as A, but they didn’t know she had two Kagunes; that alone increased the danger of facing her and thus, her rate increased. I don’t think we knew Ayato’s previous rate, but I guess his rate is so high due to his habit of massacring ghoul investigators and civilians.