Most efficient method to turn a gear extremely slowly?
Steppers require current - and power, due to I^2*R losses) to hold a position. Power off, they free-wheel. What kind of efficiency do you need? A stepper might not be the most power efficient.

An escapement permits or causes a gear to index one position and prevents it turning any farther. In a clock, it permits a weight-driven or spring-driven shaft to index. You'll probably want one that drives the shaft as well, probably with a solenoid. 

Another option that comes to mind and doesn't need an Arduino, is a clock motor. Gear down the hour hand 2:1 and you're done. Find a 24-hour clock motor and even that gearing will have been done for you.