You'll find that the most common answer is going to be a Sig P226. For some reason, it seems to be a couple decibels lower than any other host. I run mine on a Glock, Beretta 92FS and AR 9mm Carbine. Honestly, to my ear, neither pistol is markedly louder than the other. The correct answer would be which ever 9mm you already have.
well thats no fun
Honestly, I will probly just thread one of my Beretta 92's. I had thought about getting a nice cheap police trade in 226, but then I saw how expensive Sig threaded barrels are...
If you decide to go with a sig, you can have the barrel threaded, and an adapter made to extend the barrel for your suppressor. I know ADCO does that work. They threaded my Beretta barrel for me. Nice work.