Peter's definitely the biggest idiot of the three. While Homer's not exactly the brightest crayon in the box, he still tries to please his family and does care about them, despite the way he seems to treat them sometimes. Have you seen how he dotes on Maggie (when he remembers she exists) and Lisa? Not to mention that he certainly loves Bart despite the disrespect he often gets from him. And he's always willing to show Marge how much he loves her.
Now Stan might be heavily Conservative, seriously closed-minded about some things, paranoid as hell, and is probably one of the most patriotic American on TV, but he's not completely stupid. It's pretty clear in a lot of episodes that while sometimes he does act selfishly, he also cares a lot for his family and tries to do what he believes is the right thing for them (even if it really isn't). Stan is a good father in that while he and Haley argue a lot and have very different views on a lot of things, he still loves her and is clearly afraid of losing her completely, and many of Stan's interactions with Steve often revolve around Stan trying to help Steve improve his life, or at least stay out of trouble. And even he will call out others on their rashness and stupidity (mostly Steve and Roger. Especially Roger). Stan does have issues with Francine, but that is a result of him letting his pride get in the way. There's also the fact that he does occasionally seem to learn from his mistakes and to be more tolerant of others. You know, he has character development, no matter how minor it is.
Peter, on the other hand, often has to have even the most obvious of things explained to him by Brian. He often abuses his family, especially Meg, in some way and he seriously fails to think things through at all before he does anything. He never listens to Lois and many times he is apathetic when he shouldn't be until someone tells him why he should care, unless it's Meg. Peter's stupidity alone is why I stopped watching Family Guy.
tl;dr: Homer and Stan are men capable of caring for their families and be there for their children when they need to be, while Peter's just a selfish jackass to the point that it's a miracle he still has a family.
_________________
My Aspie score: 163 of 200
My NT score: 45 of 200
I am very likely neurodiverse.