Unlike desktop computers, laptops aren't grounded and there are no parts which are 100% safe.
As for USB, usually there are static protection chips installed in computers which are supposed to prevent destruction of the port, but they aren't meant to be shocked all the time.
As far as I know, but I could be wrong, in normal operation USB ports are components which are most exposed to static electricity and they should have best protection. Another part which should also be protected is the 8P8C network jack.
So safest way would be to touch outer shield is USB cable. Since it causes glitches on the port, the amount of electricity is considerable and will probably soon render ports unusable.
I know you specifically asked NOT to receive adivce on how to prevent static buildup, but I feel I must mention anti-static mats. Basically it's mat which plugs into something grounded such as ground pin on a power outlet. Place the laptop on top of one of these and discharge yourself by touching the mat before touching the laptop. Its advantage over wristband is that you don't have to physically ground yourself and are free to move. They can be purchased for $10 to $30 depending on the size an are definitely much cheaper than repairs of damage caused by static electricity to the computer.