As it is I voted for Stradiots because they're near perfect: fast, lances, maces (if only they had a little shield). The ludicrous amount of time and money it takes to obtain them in the campaign though makes them virtually impractical.

If HAs were allowed I'd pick Vardariotai. I found Dvor Cavalry awful to control. I really can't be doing with any barded or plated horses that don't have 2 HP to compensate for their sluggishness!

Feudal Knights and the like just seem to get creamed after the charge wears off simply because they can't get their backsides out of the melee quickly enough.

I'd much rather have medium cavalry like Demi Lancers - they're a nice little bonus when your English city hits the huge level. I also really like the Italian Cavalry Militia. Available in cities early with a Merchants' Guild, cheap-as-chips, heavily armoured riders and medium speed.

A big mention also for Mailed Knights. They're literally the work horses of Western factions. Some might think their ubiquity would make them a dull choice, but I love them as a reliable medium/heavy unit.

I couldn't get on with Huscarls simply because they lacked lanced charges, but I agree the Norse War Clerics are pretty cool with their blood red barding and gold-crowned riders.