1. Are you already experienced? In that case, try out all the gyms in your area and choose the one you like best (with your experience, you should be able to tell which coach works best with you)
2. Are you a beginner? In that case, try out all the gyms in your area and choose the one you like best (you don't need the 'best' gym and coach as a beginner, just a gym that you enjoy and want to spend a lot of time at)
As for legit-ness: Most 'mma gyms' are plenty legit for beginners and showing you the basics. The nonsense martial arts ones have mostly been weeded out.