Aspidiotai Hippeis. Best possible heavy cavalry? No. But you can build recrutation for this unit across the bigger part of the map. Good enough to engage any other horsemen and you don't have to walk them across the one third of your kingdom for retraining.
TBH, a few units on the campaign map needs to be toned down either in their power or availability. Sarmatian cataphracts, foot/horse Thureopherontes archers - for an example. It's far too tempting to eventually create an army mostly with these.