What power adapter and cord are best for charging Mophie Power Station Prime 20?

Start with your PowerStation User Guide. This should provide a recommendation for input power from a USB-C Power Source. Using a more powerful Power Adapter (this likely being physically larger and more expensive) will ensure that the PowerBank charges at is maximum supported charging rate, but offers no other benefit.

The online specification that I can find for your Power Adapter states that it supports 18W output power when using USB-C Power Delivery. In absence of additional information, it is probable that your PowerBank will have a similar charging rate. As such, a 20-30W USB-C Power Adapter that supports USB PD (Power Delivery) will be adequate to charge your Power Adapter at its maximum charging rate. Most USB-C charging cables are designed to support at least 30W.

the way charging works is that the device you are charging will draw the amount of current (amps) it was created to charge with

watt = volt * amps

so if your power station came with a say 30watt ac charger then getting a 30000000watt ac charger will not make a difference, it will just leave a charger with a whole lot of untapped capacity.

so use the ac charger it came with or get one around the same watt as the one it came with.