They all have problems at some point or another. That said, if you can get a hold of a low mileage 05-07 Chevy with a Duramax/Allison tranny combo, you will have a very reliable setup for many years.
I wouldn't recommend a Ford 6.0 or 6.4 Powerstroke UNLESS you are willing to make your own repairs, and believe me, you will be making repairs and are prepared to spend some money to make it right if the previous owner did not.
Dodge, I can't speak to except that a Cummins is hard to beat. unfortunately, the trucks have tranny and tie rod problems quite often. If I have the chance to transplant a Cummins into my F350, I will. But, engine swaps aren't something your average TCer wants to do unless he's a motor head.
Gas engines makes selection a lot easier, at least for me. The newer Ford 6.2 is proving to be an outstanding engine. If you can find a V10, that'd be good, but expect to get about 11 MPG unloaded, and about 7 loaded.
I can't recommend a GM or Dodge in a gas configuration. That's just me.