When talking with other cyclists, particularly other fixie riders the subject of gearing often comes up. My usual ride has an odd but very specific gearing: 47 up front, 19 in the rear. Why?
The gain ratio (47 / 19) is just a hair under two and a half (2.47). Good for going up (and down) hills in San Francisco. 47 and 19 are both prime. This distributes the wear pretty evenly over the chain and sprockets. Any given link on the chain will hit the same tooth on the chainring every 893 revolutions, which is approximately once per mile with 700c (~27 inch) wheels.
Empirical evidence leads me to believe this ratio is as close to perfect as you can get for San Francisco, for a brakeless everyday fixie. It's equally good for banging around town or a paradise (Tib) loop.