I really like having to make a boob tier list, because I believe it exposes the fact that people have a misconception thinking that bigger boobs = better.
I think people don't realize that it's not about chest size, but how it looks, the shape/form and how hot you perceive it to be combined with the body shape, and it just so happens that a bigger chest is easier to pull off in most cases, but it doesn't make it more attractive per say.
Basically with big boobs you can't really go wrong, but a smaller chest can get a much more erotic and devastating result.
That being said, I have looked at all of the girls' chests for research purposes, and this is my professional opinion:
S = Hot as the Sun
A = Very Hot
B = Hot
C = They're alright
D = I don't like them