I would recommend only using the minimum agility needed for your riding difficulty (so 15 agility for a destrier). Having 6 or 7 riding on a destrier won't see much difference versus 5 riding (in speed and maneuverability). However, you will notice a big difference between 5 powerstrike, and 7.
My horses of recommendation would be these 4 (depending on how much riding/agility you want to invest): Rouncey, Destrier, Courser, Arabian.