Contrary to what everyone else say, I think despite Bheeshm having had let go his personal happiness for his Pratibha, I think practically he was always the king behind the throne at least till Vichitraveer and Pandu were the kings, and even during the Dhritrashtra's rule till Duryodhan reached adolescent and started affecting his father's opinion.
Strangely it had nothing to do with him being efficient but only the fact that he wanted to rule but that Hastinapur always had inefficient kings
Despite Vichitraveer being elder, it was his decision to make Chitrangad the king (no doubt Vichitraveer was not efficient enough), but after his death we had Vichitraveer on throne, post that it was only Bheeshm who took all the decisions. After Vichitraveer's death although Satyavati was the queen, still the ruler in real sense was Bheeshm, he ruled indirectly till Pandu and Dhritrashtra weren't back from Gurukul, even after that it was he who threatened Gandhaar with the option of war it they didn't agree for marriage alliance, he remained the decision maker, had better spy system than the king. Maybe he didn't have the title but I can't think of any reason why he wasn't the king
