I said Sonny & Tom. Especially in the novel - when Tom is set free by Sollozzo, it is Sonny who rushes up to see him. Michael thinks that Tom is closer than Michael can ever hope to be with his own flesh & blood brother.

I agree with Plaw - Michael was never close with anyone once he came back from Sicily. He doesn't really get tight with Connie until the 3rd film and I think that's only by default. By default, I mean that she's the last one left of Fredo, Sonny & Tom by the time of GFIII.