The Vale arguably has the freshest army. They stayed out of the War of Five Kings and have a strong position. I doubt the battle with the Boltons taxed them greatly. That being said we don't know the size of their forces.
Good thing for Dany they are on Jon's side. Dothraki may have the numbers but the Vale knights are better equipped and quite capable on horseback.
I really want to see more of the Knights of the Vale in action.