I've settled on a Ram CTD 2500 or 3500, planning to buy later this year or early next year (kind of enjoying no payments now and no immediate need). Your mileage may vary, though.
I didn't really look hard at the GM trucks. I've never been a fan in general and they've obviously had some recall issues as of late. My specific dislikes are the IFS setup, the crazy DEF tank location and the styling (totally subjective). Good drivetrain, though, and very fast.
I thought I'd probably get a Ford but I've read a lot about saggy suspensions and after driving one you can definitely tell the overall platform is dated at this point. It just kind of has that old school jiggly truck feel to it. I'm also a little leery of Ford's diesel motors after all the issues they've had over the years, and they seem to have a drinking problem - I work with a guy with a 2012 that's never gotten decent fuel economy. They're very nice looking trucks, though - my favorite of the three in terms of exterior styling.
So why did I end up settling on the Ram? Stout frame, stout axles (including SFA), and of course the Cummins - which with due respect is the overall best of the three diesel motors (again my opinion). It is slower from a stop than the other trucks but once you're moving it can keep up fine, but even from a stop it isn't what I'd call slow. It still has a diesel sound to it so I like that as well, plus having a DEF gauge instead of a warning light is nice. It seems to get pretty good mileage and the emissions setup finally seems to be dialed in pretty well. The 2500 coils ride really nice, although the payload is a little skimpy which is why I'm still considering a 3500. IMO it has the best interior at all trim levels.
Overall the Ram just checks more of what I consider to be the important boxes than the other two, but I do think you could buy any one of the three and be totally content.
2015 Cummins Ram 4wd CC/SB