Any military officers in the family (yourself, partner, parent)? If so, USAA. Otherwise, Geico.
Geico is a direct writer so they have less overhead because they're not paying out as much for local agents. Rates are often cheaper for that reason.
If you have activity on your record (tickets, accidents) then Geico might not hold as much savings.
You might also want to visit http://www.jdpower.com (consumer review site) to see how the various companies compare.