I don't know about "best", but this is the one I have found to be the most convenient and easy to use. It plugs into, and is powered by, the OBD II port on the Karma and connects to my Iphone 5s via Bluetooth. I can use it to download codes and turn off the MIL. I can also use it real time to monitor a fair amount of data using its basic app. Since there are no cables to mess with, I just leave it plugged in all the time.
Click on the photo below for more information from the manufacturer.