Constantine V desrves to be the in top 5, if not no1. He secured byzantine frontiers agains the arabs and bulgars decisively beating them in numerous battles. Put the empire on the offensive, giving it breathing room. He also repaired Constantinople after a huge earthquake. Constantine formed the tagmata, the emperors personal retinue that was always on the ready, consisting of the empires best soldiers. These troops alloud quick mobilisation and counter attacks against the empires enemies. The only flaw of his reign is his harsh iconoclast policies, that forbid the venitration of icons.
Overall he was a great emperor fixing the empires problems and setting the stage for the macedonian golden age.