Factory barrels are what I always look for. I've got a factory metric threaded barrel for my M&P 9, shoots great. I've got two 9mm suppressors right now, a Gemtech Multimount and a Liberty Infiniti. Didn't think the Multimount had a first round pop until I shot the Infiniti, although the Infiniti is looonnnnggg. Through online reviews, the SilencerCo Osprey and AAC Ti-Rant get the best reviews. I'm considering a Ti-Rant 9 for my next can, not sure yet. Also, if you plan on suppressing anything larger than 9, you can always get a 45 can and shoot 40 and 9 through it with very good results. Good luck.
M&P 9 factory threaded barrel w/ Gemtech Multimount
w/ Liberty Infiniti
w/ SilencerCo Osprey 40
w/ AAC Ti-Rant 45
and length comparison(left to right Ti-Rant, Osprey, Infiniti, MM)...