Question: what is the best natural gas tankless water heater?
Answer: Rinnai V75iN Natural Gas Indoor Tankless Water Heater (7.5 GPM) “a good choice for the average household.” ... 
Rheem RTGH-95DVLN 9.5 GPM Indoor Direct Vent Tankless Natural Gas Water Heater. “one beast of a tankless water heater!” ... 
Rinnai RU199 Sensei SE Tankless Hot Water Heater.