Simply put, Rinnai makes the best tankless water heaters on the market. They also provide some of the best training and technical support of all water heater manufacturers. Rinnai has been know to maintain very high quality standards and keeps their manufacturing and customer support in-house.
Another trusted name in the tankless water heater in Navien. They make the most efficient tankless unit on the market at 98% efficiency. The Navien tankless unit has great output when compared to other units of similar size. You can also vent the Navien units farther than many other brands.
Noritz was introduced to the US tankless market just over ten years ago but have been making water heaters for more than 60 years. In 2006, Noritz became the first tankless company to offer ASME Certified commercial units.
Eternal Tankless Water Heaters have had more issues than any other tankless manufacturer in business. The most common issue with the Eternal Hybrid is the failure of the mixing valve. Even when a unit is under warranty you will usually have to pay for expensive shipping cost if you want your hot water restored.
Rheem is one of the biggest names of all water heater manufacturers. They are also one of the leading tankless dealers in the US. The Rheem units are priced lower than most other tankless water heaters on the market.