Filtrete 10x20x1 Air Filter, MPR 300 MERV 5, Dust Reduction, 1 Filter

As a property manager, I'm very sensitive to getting a filter that puts the least amount of stress on the fan motor while keeping dust/dirt from entering the system and causing problems at the evaporator (one goal is prolonging the HVAC system life). This level of filtration accomplishes this goal well for a decent price. I think other brands with this same level of filtration are fine too (and probably cheaper). If you're willing to spend the considerable extra amount of money to filter allergens, mold, etc., you'll need filters with a higher filtration number (or MERV rating), but these will put more stress on the fan motor. I do not suffer from allergies and I'm not unusually sensitive to mold, but there is a lot of science and my own empirical proof from professional (and expensive) mold testing experts I hired, backing up the fact that even with these "dust and dirt" level filters, allergen and mold concentrations are a fraction of their outdoor concentrations. So unless you have an active mold source in your home, I highly recommend sticking with this level filtration. Your wallet will thank you in the short and long term. Lastly, few people change their filters regularly (about every three months), but it's key to do this for efficient operation (lower electric bills) and longer system life.