What's the best AC Filter for indoor air quality?

You want at least a merv 11 or better. Merv is the filtering rating until you get to the HEPA 99.9 filters for operating room and highly allergic filtering units. You won't find them in less the 4 inches thick and they are "Pleated". They look like (WWWWW}

The best of the best is the 3M Filtrete 1900 Maximum Allergen. They are changed every 3 months.