I can only get my hands on rough numbers, so take these with a slight grain of salt.
I can tell you this: The Ford Super Duty is definitely #1. They sell over 350K of these annually (40% of all F-series). The 'Stroke is somewhere around 60% of Super Duty sales (200K diesels per year), but that is likely to change with the new 6. 0.
The Ram HD is about 35% of all Rams, which puts it at around 140K units. That translates to over 110K Cummins Rams per year.
Chevy/GMC may actually beat the F-series in total units for the first time in decades, thanks to heavy incentives. I don't have their HD or diesel splits though.