I am a female so not a truck person, however my husband says a Ford powerstroke diesal is the best buy. In fact, if you are pulling weight like a boat, RV or trailer with bobcat or something heavy the F-250 series is one of the best. I don't think they get more than 15-16 mpg though, but not for sure on that.
You are correct about the mileage. I know that 7years ago the power stroker was considered the best. I am not sure about the past couple years. I think everyone is using different engines.