Name brand and tire rating. Reviews if there are any available.
IMO, if you are to go off of one thing alone I would say tire rating. Most people, unfortunately, don't understand this.
The standard tire rating has three parts: treadwear, traction, and temperature. You can read about them here:
http://www.discounttiredirect.com/direc ... adwear.jspPersonally, I want a tire with an "AA" traction rating. That is by far the most important thing for me.