When trying to answer the question of what is best there should be a list of disclaimers, because nothing is 100% best 100% of the time. For this question, we are addressing commercial HVAC applications for non-critical facilities like schools, office buildings, warehouses, restaurants, shopping centers, etc. With that in mind, there are a few reasons why MERV-13 is the best: 1) Particle Removal
The most important aspect of an air filter is what particles it removes from the air. Since 1999 we have been testing air filters to ASHRAE Standard 52.2 and assigning filters a MERV (Minimum Efficiency Reporting Value) based on a given filter's ability to remove certain particle sizes from the air.
The problem with measuring filters per ASHRAE 52.2 is that it makes no correlation between particle removal, and the particles that are in the air. This is why a new Global Standard ISO 16890 was developed. After years of researching what particles are in the air (known as Particulate Matter or PM) 16890 synthesized the research on PM and created a test method where filters would be tested against what they will likely face while in operation in a typical HVAC system.
The results of this new testing method were eye-opening to the American air filter industry - so much so that there has been a big fight by many air filter manufacturers and sales reps to block implementing ISO 16890 in the United States. What we discovered is that the most common air filter efficiency, MERV-8, removed less than 5% of all particles in the air. MERV-8 is so poor at removing particles that it didn't even register on most of the new ISO rating system. What's just as bad is that MERV-11, which has long been considered High-Efficiency, was found to remove well under 50% of the most harmful PM1 particles. It is not until we get to MERV-13 that we reach 50% removal of the smallest, most harmful, and most abundant sub-micron particles.
2) Fit in Current HVAC Systems
If a filter need expensive retrofitting or replacement of your HVAC systems, it isn't useful. That's another big advantage of MERV-13. There are more options for this filter than any other - so it is unlikely that you will not be able to find a MERV-13 that will work in your existing system.
This availability also means that there are multiple designs to choose from that have different prices, dust holding capacities, and energy use considerations. The growing awareness and demand for better Indoor Air Quality has forced a lot of research and development by filter manufacturers into these high efficiency filters.
3) Superior Equipment Protection
Cooling coils, fans, and blowers all have to work harder and use more energy as they get dirty, so it follows that the more particles you can reduce from the air stream the longer it will take for coils, fans, and blowers to get dirty - making them more energy efficient. In addition, all the sensors and monitors put into equipment today is also sensitive to dust and particles so by increasing the particle removal and keeping the air clean it will keep all of those things from fouling and reporting bad data.