I've been truck shopping for the last 12 months so I've looked at quite a few. I almost bought a 2014 with just under 30K miles but it got away from me. The ride quality was very good. I attributed this to the tires that were on the truck (Pirelli Scorpion STR 275/55R20) and the fact it had mechanical load-leveling shocks. I believe the load-leveling rear hydraulic shocks provide improved ride smoothness.
I've driven several 2015-17 models. The best ride quality is on trucks that come with Continuously-Controlled Damping shocks (CCD). These are available only on Limited, King Ranch, and Platinum models that come with 22-inch wheels. The ride quality is simply amazing. Once you experience CCD, all other trucks seem to ride "rough" in comparison. You can really feel all the little bumps with the normal suspension. The bigger the wheels, the worse it gets. CCD smooths out the bumps considerably (at least on the "comfort" setting).
EL models have a longer wheelbase and so offer the smoothest ride. Trucks with 18-inch wheels have the most rubber between the wheel and the road and so offer a smoother ride. Bigger wheels mean you can feel bumps more easily.
2015+ models have electric steering that feels quite different from older models. Also, the new Ecoboost is a lot noisier than the 5.4 V8. I'm not sure if they installed more soundproofing in the firewall starting in 2015, but I can really hear the difference.
I spent some time talking to a Ford tech recently and he had great things to say about the Ecoboost. It is solid and reliable. I was concerned it might cost more to maintain in the long run, but he said they had never replaced an engine and did not often have to do major repairs.
What I am most concerned with is getting a truck that is not rusted underneath. Be sure to get underneath and personally inspect any truck you plan to buy. I had a deal on another low-mileage 2014 that fell apart when I found significant corrosion on the drive line and underbody. The rocker panels in particular are known for rusting out, so once that "cancer" gets started it can be very difficult to stop.