I'd go for the fastest/most maneuverable. Typically any time you ride through infantry you can be stopped by a pointy polearm, if that happens your armor/hp on horse isn't going to be doing you a lot of good.
Personally for 5 riding skill, I think the destrier is the best horse. If you want to go up to 6 riding, then courser, and the best horse is the arabian horse (that's just my opinion as an 8th gen cavalry lancer). As cavalry the point is not to have your horse get hit, and the best horse (again, IMO) is the horse with the best speed and maneuver.
If I had to choose an armored horse, it'd be the warhorse.