This is indeed a hard question, and many great names have been put forward to fill the slot. However, I would have to agree that Octavian was the greatest of all Roman politicians. He tied together a Republic torn and shattered, with the support of the conservatives, and the name of Caesar - truly a feat at the end of the Republic! He had himself named Princeps, and forged for himself a position that would last hundreds of years.
Definitely Octavian. And definitely not Caesar - even as a general, I think he's overrated