MERV stands for Minimum Efficiency Reporting Value, and ranges from 1 up to 20. The higher the rating, the greater a filter's effectiveness. It generally doesn't take more than a few dollars per filter to jump several grades higher on the MERV spectrum. I would say that, as a rule of thumb, go with a microallergen filter, which usually has a MERV score of at least 11. However, if you want to hone in on more than just ratings, there's a whole line of Filtrete air filters that can weed out various particles depending on your intended result, such as reducing odors or allergens.