I was gifted a $50 phone back up battery that came in a med sized notebook case so i charged it and tossed it in the truck..and immediately forgot about it. While traveling, my phone charge cord failed and i remembered that there was a usb cord and adapters with the battery so I pulled over to look for the cord. In the case was a pair of small battery clamps which i thought were a joke. When i went to restart, the truck just clicked and i knew the 5+yr old battery was done. Stuck in a parking lot, i opened up the notebook case and had a chuckle when i grabbed the battery, clamps and cord. Much to my amazement, the little back up battery, about the size of a smart phone, started the truck with ease...and still charged my phone...so the moral, i am not sure there is a best, they are probably all from china...but even the cheap ones work