I have no freaking idea what the vangaar unit is or whatever is called , but maybe add Cataphracts :) I love that unit so much. 

Varangian Guard was a unit of at first Rus (probably still mostly Norse) and then Scandinavian/northern mercenaries serving the Emperor, after locally hired guards proven to be an easy way to get yourself assassinated if you got into disagreement political in nature. I don't really want to argue whether one is better or worse, but Varangians are a quite unique unit, while Cataphracts in some form were utilized all around the region and by the era the Byzantines enter the game making them just a reskin for knights would be (is? I'm not really sure how do the knights look for Byzantines) quite okay.

Here, let me Google that for you.

"The Varangian Guard (Greek: Τάγμα τῶν Βαράγγων, Tágma tōn Varángōn) was an elite unit of the Byzantine Army from the tenth to the fourteenth century. The members served as personal bodyguards to the Byzantine Emperors. The Varangian Guard was known for being primarily composed of recruits from northern Europe, including mainly Norsemen from Scandinavia but also Anglo-Saxons from England.[1] The recruitment of distant foreigners from outside Byzantium to serve as the emperor's personal guard was pursued as a deliberate policy, as they lacked local political loyalties and could be counted upon to suppress revolts by disloyal Byzantine factions.

The Varangian Guard not only provided security for the Byzantine emperors, but also participated in many wars, often playing a decisive role, since they were usually deployed at critical moments of a battle. By the late 13th century, Varangians were mostly ethnically assimilated by the Byzantine Greeks, though the Guard remained in existence until at least mid-14th century. In 1400, there were still some people identifying themselves as "Varangians" in Constantinople."

Much more interesting than cataphracts.