You asked a fair question, McRod, but I don't believe that anyone other than Vito (and later, Michael) had any real decision-making power. Tom was closer to Vito than either Tessio, Clemenza or Sonny--but that was strictly defensive on Vito's part: He had to give orders to one person at a time so that, if anyone turned rat, there'd be no witnesses (and no felony prosecutions). My take is that Vito listened to Tom's advice, but Tom didn't make any decisions on his own. The only orders Tom gave to Tessio and Clemenza were Vito's, conveyed through Tom. And when Vito was disabled, Sonny was the acting head of the family--but he made only tactical decisions that would hold the fort until Vito recovered. Tessio and Clemenza took orders from Sonny during that period.
As for Michael: He was the hyper-controller. Tom wasn't even his consigliere as Tom was with Vito.