The vast majority of heat exchangers are counterflow. Parallel exchangers are sometimes used when you want to stabilize and control the actual outlet temperature instead of just maximizing heat transfer, since the gradient at the outlet is much less.Many years ago I thought I had invented a method to efficiently distill a continuous flow of water.
It turned out I had "invented" exactly this (using latent heat from condensation to preheat the incoming cold water).
Counterflow heat exchangers are used in desalination plants and many other industries to capture what would otherwise be waste heat.

