What is the best electric heater for a 15 x 30 above ground pool?

The volume of a pool this size is 12,000-18,000 gallons. This requires a heater that can put out about 150,000-175,000 BTU/hour. No electric heater is efficient at that range so you'll probably have to use a gas or propane heater. You ''could'' get by with a rather inexpensive 100,000 BTU heater in the $600-$700 range, but you will have to plan your swimming in advance because it will take that unit about 24 hours to raise the temperature of the water 15 degrees. ---If you are lucky. Other systems can do it faster, but it will cost more.