What is the best / recommended voltage to charge your iPhone 15 pro max?

Voltage doesn't matter to the iPhone itself. The charger converts AC to the current the iPhone will take. Apple USB power adapters are designed for use with power sources rated to provide 100V AC to 240V AC at 50Hz to 60Hz.

Or do you mean wattage? If so, the iPhone can handle pretty much any charger that's certified to work with Apple devices. If the charger provides a higher wattage then the iPhone accepts as a maximum, the iPhone will in essence just ignore that extra current.

If neither are what you mean, please clarify.

Regards.

You will need a 25-30 Watt charger for good performance with the iPhone 15 Pro Max.

You can use up to an 85-100 Watt charger or even higher if you want, but it will not charge the iPhone's battery any faster than the 25-30 Watt charger.