The type of filters you should install depends on the thickness of the filter that your air conditioner is compatible with.
One-inch thick filter capacity - An air conditioner that accepts only one-inch thick filter can use MERV-5 or MERV-6 (minimum efficiency reporting value) fiberglass or pleated filters. These filters are about 25 percent effective.
More than one-inch filter capacity - An air conditioning system that can accommodate more than a one-inch thick filter can use a MERV-11 filter that is around 85 percent effective.