In my opinion--your's will vary--
Avoid 2011 first year transmission troubles. The 6.7 has been "fairly" trouble free. Each year gets a little better, but a little more torque and HP so loaded a little hotter. Ford got a bad rap for not paying for high pressure fuel pump and injector failures that showed evidence of water in the fuel. GM was paying as they came out of bankruptcy, and Ram claimed more reliability by using the CP3.. However, to keep up with torque wars, RAM has now incorporated the CP4 pump. Reality is that water in the fuel is not Ford's problem. I always felt that I would replace the high pressure fuel pump if I bought used just for insurance against abuse by a prior owner. Our 2012 has been fairly acceptable for reliability. They still fight the "plugged" EGR cooler problem with the earlier 6.7 models. Later ones (about 2015) started running the hot radiator loop through the EGR cooler, but don't know if that has helped reliability in regards to plugging. And, of course, the 2017s eliminated rust with the aluminum body.