What is better air filters of good quality, an ozone generator, or an ionizer for good healthy air?

My friend just moved to an apartment and is concerned about off gassing and indoor air pollution; we are in a friendly debate as to what strategy [or combination approaches] is best to address her concerns. Neither of us are engineers or technical. I know there are plants [and I have that list] that help clean and purify indoor air. Also the natural allergens in our geographical area are generally high.