So... you are asking what the best cavalry in the early era is?
I could try to answere this question...
OR
tell you my poinion on something completly different you did not ask for... like the others!
So here it is => Elephants are the best cavalry!
They are not light... and not available until extremly verly late... you cannot even really recruit them as they are merceneries... but I like them so much I give a crap about the OP and talk about this!
No... seriously:
It is hard to tell which light cav is best... the "better" the unit is the slower it replenishes.
At the end it is the "feeling" that makes you like a unit more than another and the best unit is worthless if you cannot replenish it!
I usually run down Ritterbruders with peasants... I pay like nothing to destroy a very rare and expensive unit!
You want to charge into your enemy: Piast Nobles ( Poland ) btw I cannot remember if they are heavy or light... or Lithuanian... I forgot the name.
You do not laugh about these chickens throwing javelins and run away then: Jinetes ( Spain and some others I think )
Horse Archer: muslim factions, every one of them has only a little difference in the game feeling, I mean in horse archers, the rest of the army differs!
Do not play Hellas!
You game will not be balanced... first you will suck... then rule ( "best" units )... then suck again because of no good firearms! I find it too easy as the units really kick ass and I stop playing when I get too big...