Depending on your destination, you will need different kinds of filters. For example, if you’re drinking Southeast Asian tap water, you’ll want a SteriPEN, which will kill (nearly) all the bacteria, protozoa, and viruses in your water. If you’re drinking water from a European stream, you’ll want a Sawyer filter, which won’t kill viruses (which you won’t find in that water), but will filter out particulates. If you’re going backpacking in Cambodia, you’ll want both. It also wouldn’t hurt to have a backup water cleaning option, like iodine, which is good to have in a first aid kit anyway.