I think, Sonny had the worst manners. Even worse than Clemenza who at least appreciated and enjoyed good food.
Sonny is eating even in important situation. It seems that eating for him is just a nervous habit: He's throwing peanuts in his mouth while his father and Tom are discussing the Sollozzo proposal. Also, in one of the additional scenes, he's eating bread while he's thinking what to do after the shooting of his father.
The way he acted in those scenes made me think that James Caan was a brilliant choice for the character of Santino Corleone.