I was recently asked to to describe the various types of water filters available out there, and how I feel about each of them. I personally prefer Gravity Filters because they use no energy, waste none of the water and clean almost everything bad out of the water. Reverse Osmosis is also good, but a large amount of water is wasted in the process. Distillation is also good, but is very energy intensive, and many people believe its unhealthy to drink water that’s completely empty of minerals.
Here’s the Gravity Filter that I personally used before I became nomadic, though I also added the Arsenic/Fluoride filters which are usually bought separately.