Well, there are three factors...the insurance company's rates, your discounts, and the level of coverage.
Mercury insurance generally will be the cheapest insurance rates, and liberty mutual will be 2nd or 3rd cheapest.
If you want the cheapest insurance rates, tell them you want the state minimum requirements for auto insurance, don't add on anything such as rental car, etc, and say you want a liability only policy. This will not put comprehensive coverage on your policy, and will reduce the term rates for you.
However, this is not a good way to go, I would say pay a good amount for your insurance, because at crunch time after an accident or theft has occurred, whatever amount you damaged or are found responsible for, you are legally liable for. So basically, whatever the insurance pays is great, but anything past that you are responsible for by law. This means that if you have a liability only, state minimum policy and total a $50,000 car, your insurance company will pay $10,000 to the other party, not pay anything for your vehicle, deny you a rental car, and the other party will take you to court for the remaining $40,000. I highly suggest getting a middle level coverage auto policy for peace of mind, it may not be the cheapest price around, but you won't have to worry about losing your house or savings account over it.