I think maybe the thread title should be changed to the ultimate fun
. The bike is only one part of the equation. Unless you live in a vacuum in a flat area a single speed is not the most efficient bike. Simplest but not most efficient.
Every rider will have a self regulated cadence that is their most efficient. Most competitive riders it's around 90-110 rpms. You'll pick your gearing on the SS to match you optimum cadence for normal cruising speed. Throw in winds or hills and the efficiency has gone out the window. Grinding along at 40 rpm or spinning at 130 rpm is not efficient. That why they put gears on bikes so that you can stay in you optimum rpm range for the entire ride.