Lots of opinions about what are now 'older' diesels, where we all like to wax nostalgic and/or defensive (in the case of the 6.0 owners ;) ) about the pre-dpf diesels and what was the best.
But the OP was asking about trucks that are only 3-4 yrs old, he isn't going to be looking at any 6.0's, 5.9's, or anything with real issues. Except maybe a Ford 6.4, which personally, I'd avoid.
For the last 4 yrs, I'd buy whatever I liked from the brand I liked. The exception being a Ford w/ a 6.4.
And lest the OP be talked in to buying someone's old, used "gem" of a diesel, I'd avoid that, too, unless you're a diesel mechanic, have designs on modifying the truck and all of that. Some of those engines might have been great and might last forever, but the trucks around 'em are getting real old. Yeah, my '06 LBZ Duramax was the best truck ever (seriously, it was, I loved that truck!), but there is no way I'd pay someone $20k for their 8 yr old 100k+ mile truck regardless of the engine in it.
Just my $.02! Good luck!