The emperors ran together worst in parts of Volume I and II. Volume III was clearer in that regard as it only dealt with Theodosius, the reigns of Honorius and Arcadius after the final split of the Empire, and then the denouement of forgotten and insignificant emperors with virtually no power for the next 25 years or so in the West under the power of Odoacer.

Volume IV deals almost solely with Justinian and his son, and then Volume V and VI (if the Hugh Trevor-Roper introduction is to be believed) really doesn't even track the emperors of the East because Gibbon focuses on East-West relations, particularly of a religious nature (and the Crusades).