There are four major expertization services operating in the United States today, The American Philatelic Expertization Service (whose acronym APES was mercifully dropped a few years ago), the Philatelic Foundation, Professional Stamp Expertizers, and Sergio Sismondo.
The American Philatelic Expertization Service is run by the American Philatelic Society. It is a not-for-profit service and actually loses money for the APS (at least it did when I was the APS treasurer, though I haven’t seen the books for a few years). It operates by using volunteer expertizers who are sent “patients” (yes that euphemism is actually used) and has all the pluses and minuses of a volunteer service. The quality of the expertizers varies widely by field, some are quite good, some “expert” only in their ability to toot their own horn. Overall, the APS certificate is usually accurate for genuineness, and not so good for quality. I don’t use them for US stamps as the next two are better. Overall grade – B.
The Professional Stamp Expertizers Certificate is a for-profit service which is thought of almost exclusively as an expertizer of US stamps. They invented the certificate grading industry and issue most of the graded certificates for US stamps. They have a fine stable of experts. Grade B+.
The Philatelic Foundation is the most venerable of American expertizing bodies. The APS was issuing certificates earlier but more better stamps have PF Certificates than all the others combined. They now issue graded certificates too. Grade A.
Sergio Sismondo is a remarkable philatelist. In the grand tradition of the great experts of yore (yes, yore!) he not only attempts to expertize the world but makes a remarkably good stab at it. I don’t use him for US stamps but he is our expert of choice for almost everything else. He’s accurate, fast and his certificates are themselves almost of collectible quality. And his prices are reasonable. Grade A.