Valentine One, with-out-a-doubt (
http://www.valentine1.com/ ). They give you 360 degree coverage, the unit is upgradeable to the latest software available and you have options to suite multiple applications.
I have it hard wired with a concealed head unit in my Boxster. I also have my SUV, wife's car and the Sienna hard wired with a Blendmount
(
http://www.blendmount.com/forumdisplay.php?f=12 ) under the rear view mirror. They're working on a Porsche mount. Whatever I'm driving it comes with me, all it takes to remove it is as simple as unplugging a phone jack, that's it. Rental car? Visor mount and traditional phone cord to the cigarette lighter.
I can't tell you how secure I feel knowing my V1 is on and sniffing the air for the smell of bacon, and then telling me when it's at.