Aria's best girl, no question. She has what you can most assuredly call a "rack": if you cannot, with relative ease and without assistance, rest something (reasonably shaped) on a pair of properly supported breasts, the pair cannot be called a "rack".
That aside, Stella's dumb. As far as sentient dragons go, she behaves with far, far less intelligence and wisdom than such long-lived beings tend to have:
-Aria loves Tama/lusts after him
-Aria is strongly protective and possessive of Tama
-Tama loves Aria
-Tama is fiercely protective of Aria; Tama's strength is effectively Aria's when together
-Tama is averse to Stella's offering: nothing but lust for mating
-Tama has already killed Stella once
-Stella thinks it's a good idea to try intimidating Aria into letting Stella NTR her