Seats are about as subjective as tires.... and almost qualify for NERPT. In any one thread, you will hear people rave about a seatmaker, while others denigrate the same seatmaker. It's all about choices.
You should probably note that Rick Mayer, Bill Mayer, Russell, Rich's, etc are *true* custom seatmakers. Corbin and Sargent are not; they are cookie-cutter seats. The latter can still work fine for some, but not for others. If you're going to drop coin on a seat, it might as well be shaped specifically for you.
Overall I agree with everything Dale said and defer to him as the master of long distance riding, but I might characterize Russell, the Mayer's, Rich's as "boutique saddle builders" because Sargent will also build you a *true* custom seat even though they also mass produce stuff.
Bill at Sargent will work with you on a ride-in and make a customized seat for you from a stock seat pan, just like other "true custom" seatmakers noted above. He won't do it while you wait, but he will generally have it done within a few weeks during the season, and maybe shorter in the off season.
Sargent will also work with you on modifications to their stock seats (which are built on their own seat pans). You can ask them to improve performance in specific areas, and they'll make modifications to try to achieve good results. No one else can do anything more than this to make something more custom.
There is a limit to just how much you can build up the periphery of many OEM stock seat pans. The seat pans that Sargent makes in-house for their mass produced aftermarket seats are more versatile and allow greater peripheral support as a rule. FWIW, it costs Sargent more than $100,000 to get set up for a particular bike with the development and manufacture of the molds. Its a major investment.
Corbin supposedly has a custom ride in capability to build a seat on an OEM stock seat pan at their California facility. I was directed to it by their phone monkeys once, but the follow up phone call did not breed confidence in the service, and I did not try it. They do not have any custom capability at their Ormand Beach facility (that's just a sales outlet).
My experience with Corbin has been that they say they will work with you to customize or fix a seat, but an awful lot of folks aren't that impressed with Corbin's ability to fix problems with their stock seats. My own experience is that when I talk to folks at Corbin, they're most comfortable taking a part number for an order, and less comfortable discussing options for fixing or developing a custom seat. FWIW, Corbin makes their own seat pans for their aftermarket seats, and if you like a firm, tractor style seat, and you're not too picky about the portion that contacts your thighs, a Corbin could be OK for you. I've had a couple of them, and they were good for me, even after 18 hours in the saddle.