Reverse Osmosis (RO)
Reverse Osmosis filtration uses a membrane which removes many contaminants from water. It is usually paired with a Granulated Activated Charcoal filter to remove chlorine and many mount under the sink and have a holding tank. The semipermeable membrane separates many contaminates (which usually have a larger particle size that water) from the water and rejects a large amount of water in the process.
The result is a waste of several gallons of water for every gallon filtered and many naturally occurring minerals (including calcium and magnesium) are also removed from the water. We used this type of filter for a long time but added trace minerals back in to the water to replace the ones that are filtered out. It does remove a large amount of contaminants but is not the best option, in my opinion.
Pros: Removes a large amount of contaminants. Many unites are stored under the sink and have a simple spigot over the counter for getting the water. Does reduce arsenic, asbestos and heavy metals. Does remove fluoride.
Cons: Wastes more water than it produces. Does not reduce VOCs or endocrine disruptors. Requires adequate water pressure to work so it is not usable if home water supply is cut off. Takes up to an hour to filter one gallon of water and filters need to be replaced regularly. Removes necessary minerals from the water.
Verdict: Certainly better than a lot of options out there and does remove fluoride, but not the best due to its waste of water and costly filters.