It`s a quite complicated task to choose the best model, as different people and pools need different functions. However, if you are looking for a heater for a small or medium pool, we recommend EcoSmart ECO 18 and Hayward CSPAXI11 11-Kilowatt ComfortZone heaters. They feature compact designs, digital temperature control, thermal auto protection, energy efficiency, and a lot of other pros. Hayward HP21104T HeatPro Titanium will suit big residential and commercial inground and above ground pools. It`s one of the best thanks to durable design, energy efficiency combined to high productivity, easy and accurate controls with displays.