best and cheapest dont go together.
bignate is right on w/ this...yrs ago, ACE seemed difficult when I was new... now after much more acquired learning & experience, ACE doesn't offer much in the way of knowledge. IFPA is about the same I understand... very basic (cheaper too). ACSM & ISSA are more difficult , yet more expensive. You can check their wesites for cert info.
I guess it comes down to your priorities.