The FTN.5 PCC Flow has excellent sound reduction and reduced backpressure vs. the standard version. Simple blowback 9mm PCCs are noticeably more pleasant to shoot with a low-backpressure can.
If you want a quiet non-reinforced (fully printed) design, the Project 27 Mashina from OKB 69 is quite good, but it's a bit of a beast from a size/weight perspective (1 lb and 10 inches long). It's also a traditional non-flow design so you'll get more gas to the face, a more felt recoil, and the action will get really dirty. If you go that route, use the 3/4x16 threaded blast chamber with a metal thread adapter since the 1/2x28 printed threads just don't hold up -- especially on a can that massive.
Be aware that even with the best commercial suppressor the gun will never be super quiet. Simple blowback actions "leak" a lot of sound when the bolt opens. Delayed blowback designs like the MP5 and Kuna make better suppressor hosts.