I suggest a scale of 0 to 5. The letter-grade scale was designed to group characters based on their power and skill in combat. I agree with Scavenger that around these parts, it implies a character’s strength as an IP - popularity, market penetration, versatility in storytelling and media. It’s confusing to try to use it to mean how much you like one. If I say that, e.g., The Question is S-tier, I’m clearly not using the scale as intended. Sherlock Holmes makes a bit more sense in that way. A zero to five scale at least has the same number of steps, if that’s important to you.