Question: what is the best commercial tankless water heater?
Answer:  Stiebel Eltron MINI 2.5 – 2.4 kW – 120V AC.  Takagi T-M50 ASME.  Takagi T-M50 NG.  Stiebel Eltron Tempra 36 Plus – 36 kW – 208/240V AC.  Eemax PR011240 Commercial Electric Tankless Water Heater – 11 kW – 240V.  AO Smith ATO-510-N.  AO Smith ATI-540H-N.  Noritz NCC1991-ODNG.