What is the best hands free device for the car? IPHONE 4s

Motorola sells Bluetooth car speakers if your vehicle doesn't have Bluetooth built in. I might get one someday.

I will second xtremecarbon's mention of Motorola Bluetooth handsfree car kits. I have a Motorola T325 and love it. It works great with Siri. It senses when you have entered your vehicle and verbally gives you the battery status and that you are connected to your iPhone using the device name. This really freaks out valets and mechanics.