Best protection and long term are conflicting requirements.
The best protection means the anode is eaten away quicker than the tank and the quicker it is eaten away the better job it is doing.
Magnesium.
Cheers,
Peter
The OKA has a copper Surejust calorifier heated by a Webasto hydronic diesel heater or waste engine heat (we have hot water after any 30 minute drive) or 240V from the inverter/solar. www.surejust.co.uk/surecal
Cheers,
Peter