You really need to specify both the motor and the truck year/body. I say this as the Powerstroke 7.3 motors have a different "best tuner" based upon whether its a SuperDuty or not.

I would say the best OBS (94-97) 7.3 tuner would be Bill at PHP (Power Hungry Performance). Others might argue Cale from BTS is better.
For a SuperDuty, some will argue the best chip tune is from Jody at DP Tuner, others might pick PHP, etc.

I have no knowledge of Cummins aside from fuel plates are easy to swap and the motors in general are power monsters.