I agree, the size of 26x1.5 is the optimal for all type of ride. I used Schwalbe Speed Cruiser during three years and can describe them as easy to roll, steady on high speed on wet asphalt, and stable to tube breakdown. Some skills allow to ride by dirts.
Now I use thinner Maxxis Detonator 26x1.25. They have better roll but they are not so stable to tube breakdown. I replace them to ride by dirts and low condition roads.