In calm winds, which runway should I select at an untowered airport?
Often uncontrolled fields will publish a preferred calm wind runway in the Airport/Facility Directory. When one isn't published the calm wind runway is usually the longest runway, which gives you a 50/50 chance at guessing right.

To further reduce your likelihood of winding up face to face with another arriving aircraft you should self-announce on the CTAF and overfly the field to observe the traffic pattern. Also, turn on all the external lights 5 miles out. If there is traffic in the pattern you should work yourself into the same pattern everyone else is using.  

If the pattern is empty my personal practice is to pick the approach to the longest runway with the fewest obstructions, and ideally one that doesn't overfly residential areas (where it's practical to avoid doing so).