HEPA air purifier is the best. Compared with noramal  filtering air purifiers, HEPA one is better in contaminates removement. As HEPA filter (the filter inside HEPA air purifiers) is a strict standard. High-efficiency particulate air or HEPA is a type of air filter. Filters meeting the HEPA standard have many applications, including use in medical facilities, automobiles, aircraft, and homes. The filter must satisfy certain standards of efficiency such as those set by the United States Department of Energy (DOE). To qualify as HEPA by US government standards, an air filter must remove (from the air that passes through) 99.97% of particles that have a size of 0.3 micrometres.  So, HEPA air purifier is much more effective in air cleaning than normal filtering air purifiers. 

And, besides filtering kinds of air purifiers, HEPA air purifier,there are also some ionizer air purifier, Ozone kinds of air purifier and UV lamp air purifier, more air purifiers here. Most of the bestselling air purifiers are HEPA air purifier. Yes, as you mentioned, the filtering kinds of air purifier need to replace the filter after 3 to 6 months, vary depends on the detailed brand and your using frequency.