Well actually the oldest ones were the best. They were much simpler and they could last a really long time. But if you're talking about modern f-150s, I would only buy the ones that have V8 engines. I would not buy the EcoBoost V6 twin turbo ones at all. And if you're going to buy one with the 10-speed automated transmission I would only buy ones that are 3 years old or newer

Personally, I wouldn't recommend any Ford-produced vehicle with that 10-speed automatic transmission, the 10R80. It was the product of a largely failed attempt at a joint venture by Ford and GM. Both corporations say they will never work together again because it wound up being a fiasco. That doesn't bode well for a transmission; it's so bad, even with repeated recalls, there are still problems with it. 

The 6R80 is relatively bulletproof compared to that 10-speed. The 12th generation F-150s (2009-2014) seem to be the best built of the more recent F-150s. They are pretty prone to molded lead frame failure, though. The transmission has an input shaft speed sensor, output shaft speed sensor, and a thermostat for the transmission all molded into one unit. You need to open up the transmission to replace when it goes out. You can replace the lead frame yourself with some basic hand tools and a torque wrench.

My lead frame acted up a couple months ago, so I went whole hog and purchased both a new valve body and lead frame assembly and had professional mechanics we've used for years put it in. You must reprogram the transmission so it can "learn" the new valve body. That requires a dealer level scanner. The transmission shifts like silk again after 173k miles.