Heat Pumps
While heat pumps are expensive, but once purchased you can say goodbye to inefficient pool heating and high monthly operating costs. Although air pumps are temperature dependent, the more popular choice that is the water heat pump is a more efficient and all-around more versatile amongst the majority of swimming pool heaters. These kinds of pool heaters also have a long life span (10 - 20 yrs) and are not air pollutant devices, despite some of them being air dependant. All in all, heat pumps are a great long-term solution.
Gas
Gas heaters are not as expensive as heat pumps and they are the most widely used pool heating system to this day. Although they heat up the water pretty quickly, their lifespan can go up to 5 years and they also emit a lot of emission into the air in order to burn off the gas. They are not as efficient, but they operate independently of air temperature, which makes them great for shorter more frequent usage schedule. Using either propane or natural gas they are definitely the best short-term option out there.
Solar
Solar panels are the least popular option out of every type of heater here, as it only works when the sun is out and in no other circumstance. On the other hand, they are very efficient and require no additional monthly cost in order to operate, as they use a renewable source of energy. They are obviously an environmentally friendly option and have a life span from 10 to 25 years, but they are not a cheap purchase at all. With solar heaters, you can't go wrong, as long as you find a middle ground.
Electric
Unlike the aforementioned systems that operate in both a small and big pool, these types of heaters based solely on electric current are best for smaller pools and home spas. Although they are pretty cheap to get, operational costs and installation expenses are pretty expensive and they re also not energy efficient at all, but unlike heat pumps, they are not air dependent. Obviously, electric heaters are not the best ones, but at least they don't pollute the air as others tend to do.