Euro-Asia Imports makes some excellent cogs as well as Phil Wood. Recommending a proper gear ratio is almost impossible. There are many factors that you have to take into account. Tire size, crank size, terrain that you ride, your level of fitness and what you want out of a fixed gear bike.

Personally I ride a 46x18 ratio. I like to spin and this ratio allows me to do so while minimizing the possibility of spinning out.