My vote goes to Sonny.

Sure, his temper was his downfall, but I think he's the big brother we all want. He's fiercely loyal and protective of his family and nothing would change that. To illustrate this think of Connie getting beat up by Carlo and Sonny's reaction to that. Vito himself would not interfere with his daughter's marital issues but Sonny could not allow his sister to suffer any physical abuse. I admire that.

Sonny had a big heart (i.e. bringing Tom home off the streets). I admire that.

He had his faults but emotionally I like his character.