Excubitor

Overall strategy
Excubitores, while slightly slower and weaker than Cataphracts, have three special perks. The first is their extensive LOS, which is much greater than what a Cataphract is capable of enjoying in the field. The second is that they have a very powerful anti-armour melee attack like some Persian units, owing to their weapon of choice - the cavalry axe. Finally, they are also capable of spotting hidden units, as per their original role as a city watch unit for the Golden City.

Summed up, it can be argued that the Byzantines don't have a good elite heavy cav unit, if we take other factions like the Tang and the Persians into account (or even the Korean Gaemamusa for that matter), but it doesn't matter since an elite heavy cavalry role is not what the Excubitor cavalry unit is for. Instead, you are meant to use them defensively -- their powerful attack, as well as their ability to spot hidden units make them an ideal unit for punishing anyone who relies purely on infantry (a lot of European factions) or melee cavalry (like the Asian ones). Excubitores are thus a "fire brigade" unit who have more in common with 19th century cavalry than the lowlier Athanatoi or Scholae &mdash; they are meant to counter enemy heavy units, as well as to guard your weaker units from enemy attacks. This is especially important if you are facing factions like the Britons, Norse and Saxons who rely on stealth units, or heavy melee units, such as the Arabs, Tang and Persians. The anti-armour bonus of Excubitores is such that in large groups, they can even overwhelm heavier units, such as Huskarlar or even other Byzantine factions' infantry (which are considered to be the gold standard for elite infantry). The only units that Excubitores then have to fear are light ranged cavalry (particularly the British Uchelwr mercenary cavalry), but these should be seen off using your own ranged units.