Glory and power
Without doubt, Tacitus expresses a sense of respect and honor for each of the three regimes he discusses, if not for the quality of their character, at least for their power. Even Nero is praised for his power, though he was a man whom Tacitus feels is despicable and corrupt. The thematic idea throughout each season of leadership is that a leader must command power, and each of these three emperors were extremely powerful leaders. Tacitus is nuanced and indirect about his political complaints against his rulers, but he is open when he praises their power.
Peace and war
In Tacitus's opinion, Augustus was the best of the three emperors, and the reason he feels that way is because Augustus used his authority as a way of bringing peace to his people. In other words, Tacitus believes that a king who can keep the peace is better than one who starts a war and wins it. The war question is spread throughout the book in different ways, but most notably, there is still tension in Rome because of their recent civil wars. Those civil wars left the state unstable and paranoid, but Augustus managed to bring peace.
Political corruption and the state
Tacitus is openly critical of the Senate. Once a bastion for political representation and high moral goodness, the Senate has devolved into a fraternity of power-hungry, indulgent men of wealth who tell the king whatever he wants to hear so they can continue in their revelry. Tacitus also notices that the emperors are becoming further and further removed from public opinion. There was such a time when the citizen's personal opinions of the emperor counted for something, but Tacitus portrays a Rome where the emperor does whatever he wants without explaining himself, and if anyone has a problem with that, bad things will happen to that person. The state is sliding into authoritarian dictatorship, especially by the end of Nero's brutal reign.
