As @GPSMan stated, Slippery is the best. Two weeks ago, finished a 1400 mi RT where all terrain was the same, weather the same, speed the same, all Interstate, etc. Did a 300 mi segment in Normal, another in Eco, and the rest in Slippery and the Slippery was 3+ mpg better. You just have to remember to press the button each time you start it up.
Slippery seems the easiest to maximize mpg without concentrating on pedals, etc WHILE IN CRUISE. I love it when I travel windy hilly 40 and 45 trip I normally do. After slowing for turns as soon as possible tap resume and power used gauge will jump quite a bit more than you probably will use but within very few seconds the cruise will be at speed and power will drop back to +/- 0 faster than you could get foot off pedal. Apparently it knows what speed it's going to and ice may not start as readily. In xlt resume needs faster speed to engage and wheel has to be straighter than the ACC on my lariat. The ACC appears to engage at slower speed and when wheel is turned making a turn but will accelerate after wheel is straightened. It can be tricky to tap resume while turning the wheel though. I can regularly get 56-64 mpg (dash) on my 15 mile 40-45 mph road in slippery cruise.