In my unit studies I have came across one cav, one cav to rule them all. If we are talking 'most useful' and cost-effective cav, then I got just what your looking for. I was suprised at first, a unit I never gave much mind to, in a faction I never played, or though I ever would play. The unit... Polish Retainers. These guys are epic, and here's why.
First, you might be thinking, that there are other units with abit more defense, so why use these? Well any cav with stats like the Polish Retainer or better has 1/3 speed(based on the testing for the FAUST guide for Retrofit), but the Polish Retainer has 2, out pacing other cav with like stats. They have the same lances, swords, morale, # of men, sheild, and defense skill as the ChivKnights, just with 2 less defense. Not sold yet? Well with an upkeep of only 130, you can field these cheaper than most cav and even most infantry.
The only major downside to these have nothing to do with the unit itself, but the fact that they are exclusive to Poland, which offers little else to make their roster good. The Portuguese and Spanish Conquistadores, The Moorish Christian Guard, and the Sicilian Norman Knights are all simular to the Polish Retainers in their speed while having good stats, they all have more upkeep(185), and the Normans have 2 less charge(which does make a difference), but at least their factions have better rosters elsewise.
Imo, the only unit that could potentially be more useful are the Turk's Qapukulu, because their side-arms have AP(Armour-Piercing) to halve the armour(not the total defense) of the victim. Making them the equivlent of RTW's Cataphracts.
As far as Cav-Archers go, I would agree Vardariotai are best, but the Russian Cossacks with a upkeep of 110 are probably better 'Cost vs. Effectiveness'. The Dragoons in the Americas Campaign are definately the best, kinda op, but are not in the GC or Retrofit, which I assume this topic is about.
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