Powerstroke or Cummins if you are buying brand new. 2016 or newer for a used Powerstroke, I believe the Cummins bad years were 09.5-14 but not 100% certain.
I made a good living at a GM dealership as a service advisor for years on just those cars alone! The problem with those transmissions (mainly) was the torque converter and the "adaptive shift learning module" or thats what we called it. You would drive it and it would upshift hard and down shift...