Some thoughts on the best pistol for a suppressor in 9x19.
1) A pistol that allows for easy changing of the sights to taller versions to see over the suppressor body.
2) A pistol that can handle more carbon fouling due to the increased back pressure associated with the suppressor.
3) A pistol that does not have a fixed barrel as it may be hard to replace the barrel with an extended model that can be threaded.
4) A pistol where barrels swaps are easy, so one has the option of the threaded barrel or a stock barrel. This may be mitigated if the threaded barrel has a thread protector to attach when the suppressor is not attached.
5) A pistol that is often used by suppressor designers.
Based on the above list, I would go with a Glock, probably a 17 or 34. Sights are there, barrels are there, and Glocks run dirty as well as any other gun. Plus, Glock support is decent, especially compared to H&K. Yes, the Glock needs a Nielsen device or LID, but lots of suppressors have been designed to work with the Glock.