The Destrier is very good but the heavier horses are just flat out better. The warhorse especially is probably the most cost effective out of all the horses regarding how it will improve your KDR.
Statwise I would pick a war horse or a large warhorse any day of the week, but after experimenting with different horses and riding points, I found destrier to be the best horse myself. I've tried large warhorse, war horse, mamluk horse, destrier, arabian, courser and rouncey all at +3, awith 5, 6, 7 and 10 riding and the destrier just feels like the best horse. Great bump damage, good speed, good at tanking, good maneuver. It's got pretty much everything.
Consequently the arabian is by far the worst of all the "expensive" horses because it dies so fast to ranged, and if you want to play cavalry at all effectively for your team you should be going for archers and bumping groups of enemies not just going for solo people alone which is where the arabian excels.
Well, if that's your view of things, then I would say that the courser is even worse. Arabian is made out of paper, true, but the high maneuver makes bumping enemies without being close to your teammates quite easy, at least compared to more heavier horses which is easy for the enemies to dodge. Makes the horse decent at supporting, even though it's fragile.