An easier question would be “What’s the most expensive filter?”, This is a very loaded question so I’ll give my take on the matter. It would be better to ask yourself “to what extent do I want to clean the air I breath?” The filters that you replace or wash monthly are considered passive in that they remove dust or particles from the air as it passes through the material. The finer that material is, the more dust it will catch. Most of the finer particles like dander and dust mites are too small to see. Now imagine those cheap blue fiberglass filters, the ones you can watch tv through. Imagine a dust mite getting blown through that filter, those fibers would be like tree branches, not very effective at catching common allergens. What’s more, much of the debris that makes it through your filter is going to stick to the wet evaporator coil and and cause some pretty nasty stuff to grow there. At the very least try a basic pleated filter and change it  evey 30-90 days as needed and if air purity is a real issue for your family there are several types of electrostaticly-charged media filters that claim to filter down to one micron and can be made to fit in your existing return.