GMA's address is a 5 bedroom 4 bathroom house. Hopefully they have a mother in law suite as well so they can have every generation of the family sitting around in the family room grading the cards while they watch Full House reruns!
The only credible graders are PSA, BGS and SGC (if resale value and respect in the hobby mean anything to you).
I personally think that CSG has the best/only chance of joining the big three.
If you want to grade cards with a legitimate grading company and don't want to hate the experience and the company afterwards than SGC is the only realistic option right now. If your pockets are bulging with money than PSA's upper tier grading services will also make you happy.
Any other grading company is fine if you just want your personal collection to take up more space in your house. Many buyers are scared of altered/trimmed/counterfeit cards in other slabs and don't respect the grades either.
As NJDevils pointed out there is no consistency in grading. In the end you need to trust your eyes and buy the card and not the grade.
Those other graders are like buying your 15 year old Nissan Sentra some brake pads on Ali Express for a fraction of what the dealership has them for and then driving on wet roads.
