Of those listed, the Cummins is the only way to go, in my opinion.
I have been hearing of a rash of broken crankshafts in the Ford 6.7. I don't have enough firsthand knowledge on the GM 6.6, but the Cummins seems to have the market for a true medium duty engine.
And by the way, it is as always an inline 6, with a torque advantage over the V-8 engines by design.
The Dodge downside used to be that the truck would fall apart around the engine, but they have really improved on that. I have a friend with a 2007 3500 4X4, and he drives it like it is a rental. It has about 440,000 miles on it, lots of dirt roads, lots of grossly overloaded trailers, and just generally hard use. I rode in it a while back, and while I expected a rattletrap, I was pleasantly surprised. There were some minor problems, but overall it was still in good shape and few rattles.
I have 200,000 mile Fords in worse shape.
On another note, I am trying out the 1/2 ton Dodge with the "EcoDiesel" 3.0L V-6. It only has 12,000 miles so far, but it runs good, has over 400# of torque, and is getting 26 to 28 miles per gallon. The fuel savings is enough to make the payment, at the rate I drive it. So far, I like it.
I will let people know what I think after it gets to 100,000 miles, and what I really think after 200,000. I remember I liked my 6.0L Ford when it was new as well.
Just remember, over its lifetime, you will likely spend more on fuel than the truck, so the mileage plays a huge roll in the total cost