There is no real best heavy cav. It all comes down to the role you need it for. For instance, hetairoi will be owned by top teir catanks, but have more stamina. Molloson Agema can run circles around both, and can do more charges in the rear of an enemy line before tiring, but are more vulnerable to enemy attacks. Hetairoi are simply the most versatile(I prefer cataphracts though).