Hello whats the best adaptor for iPhone 12? Isn't it better to use the 5 watt charger for battery health?

Battery health is not related to the charger you use (if it's classified as compatible by Apple), since modern iPhones get the correct/right amount of power from the charger. Battery health is related to the charging cycles are applied to your device.

A 20W charger is recommended for an iPhone 12.

See this support article:--> iPhone Battery and Performance - Apple Support

Regards

Giulio

Take a look at this useful user tip:-->When to charge your iPhone or iPad - Apple Community

Regards

Giulio

The 20 watt charge Apple sells for $20.

Charge it when not in use. Day and night. You can not overcharge it.

My pleasure :)

Giulio