None of them.
Plants do purify a tiny amount of air per day. So it is technically true that plants purify the air.
But the reality is that they do not purify enough air fast enough to make any difference in your indoor air quality. Simply opening the door a few times a day is going to replace more air in your home than all of your plants combined can purify. And if you have a ventilation system, then you are exchanging air in the home at a scale that renders the effects of plants totally irrelevant.
There is a genetically modified plant, Golden Pothos (Epipremnum aureum) which does a much better job at purifying the air than any other known plant. It has been shown to actually have a measurable effect on the air quality in a room.
However, most news articles and blogs are not reporting is that these results were only obtained when the plant was set up in a glass tube with air forced through the tube by a fan 24 hours a day.
So again, the claim is technically true, but certainly not true in the way most people are picturing it. Even a Golden Pothos is going to have negligible effects on air quality in a ventilated room.
You might see a small improvement in an unventilated room if you get a fan blowing over the plant all day. And remember, this is a special genetically modified plant engineered specifically to purify air. Its not your typical Pothos.
Effect of houseplants
There is one real effect that houseplants can have on your indoor air, but it has nothing to do with purity. Having enough plants, especially ones in large pots with plenty of soil, can have an effect on the humidity level in your home.
Potted plants lose a lot of water to evaporation, which is adding moisture to your air. This can be a good thing if you have dry air, which many people do have, in the winter especially.
If you take it to the extreme, then you can actually cause excessively high humidity levels that promote the growth of mold and fungus. A well ventilated home will not have any problem with this unless you are basically using it as a greenhouse.
There are plenty of great reasons to grow houseplants. They can beautify a space, bring a touch of Spring to Winter, and they’ve been proven to improve our mood. But if you’re buying them to improve your air quality, then your money is much better spent elsewhere.