I've heard ACE is the cheap and easy route. I think it's the fastest one that is still respected by most institutions.
I did ACSM which is supposedly the hardest, along with NASM... if you know your stuff you could probably just walk in and take the test. They WAY overrate how hard those things are, and are purposefully vague / misleading about what, exactly, is going to be on the test, so that you'll buy all their books.