for 9mm I hve tested the following
Unique
Universal
HP-38/W231
AA7
Clays
Power Pistol
CFE Pistol
If I had to pick a powder it would be universal. It was one of the more accurate powders I have used, and it burned fairly cleanly, unlike many others.
Specifically for a 16in carbine.
Based on my experience loading for 44spl in a 20in henry, and a 9mm 16in Ruger PC carbine.
Short answer, Power pistol, or Unique (possibly BE-86). Other powders I tried, Especially CFE Pistol, acted weird in long barrels. Best guess is power pistol and unique produce more gas volume by weight and keep the pressure up in the longer barrels. But that's just a guess. But I tested all the powders I listed plus a few, and in a carbine Power Pistol and unique were the 2 stand out performers.