#1: Boston fern.
Boston ferns remove more formaldehyde than any other plant. They're also highly efficient at removing other indoor air pollutants, such as benzene and xylene (these are components of gasoline exhaust that can migrate indoors if you have an attached garage). The downside is that they can be finicky. You need to feed them weekly in seasons when they're growing, monthly during the winter, and they like to be watered regularly. Depending on the humidity and moisture levels in your home, you may need to water them or mist their leaves daily. Another good fern for formaldehyde removal is the Kimberley Queen, a larger, broader-leafed fern than the Boston fern. It's also a much better choice if you have particularly dry indoor air, thanks to a high transpiration rate (the rate at which water evaporates from leaves). Wolverton writes that they're "one of the best natural humidifiers of all houseplants tested."