I have a 6" S&W 622 that suppresses decently and want to know if anyone who has this pistol has compared it to others. If so, which was quietest?

Bueller?

Not sure what you're expecting.  It's as good a host as any.  Guns with open breech faces like the Colt Woodsman, etc. will be SLIGHTLY louder.  Enclosed breech pistols like the MK II and S&W 622 series will provide SLIGHTLY better performance.  Adapters are all you need to worry about and they are available, IIRC.  I got mine from EWK and I like the stupid-simple hex-key attachment method of that model.  Were I to pick a perfect host, there isn't one.  The MK III 22/45 lite series are good.  The M&P 22 compact is another.  If you can handle the silly grip, the Ruger SR-22 threaded is another good choice.

I like my 422 but I think my M&P22 is just as good, maybe a little more accurate.