MERV 8 (~$8): pollen, dust mites. MERV 11 (~$15): pet dander, mold spores. MERV 13 (~$25): bacteria, smoke, fine particles. Higher MERV = more restriction = harder on your blower.

Most residential systems handle MERV 11 fine. MERV 13 requires confirmation from your installer that your blower can move air against the higher resistance.