I voted Tom - Sonny because of their constant arguments during GFI. Although they were constantly onto each other, one can tell they'd be the first in line to help the other out when in danger. However, I could have easily voted Connie - Fredo. This relationship is detectable in Part II as Michael's about to whack Fredo, Connie comes forward, in Fredo's defense. They were two members who were kind of outside the family's affairs, giving them something in common. I imagine while growing up, Connie and Fredo spent some time together, while the others were off involved with something.