The problems to obtain clean water come from so many different directions. Inorganic constituents, such as heavy metals, Micro-biological, Pesticides, volatile organics and radiological such as radon 222. A person needs to look at those problems likely to be encountered on any given trip. Even then, some of these problems are difficult to remove, shy of complete distillation.
Boiling and chemical treatments will kill the biological entities but do nothing for most of the other problems. Even in high mountain springs, water may contain naturally occurring inorganics such as arsenic or radiologicals such as radon. To simply "filter" the water is not enough to remove these inorganics or even some of the biological viruses.
In my opinion, the safest method and simplist is an Iconic Adsorption filter system which uses activated carbon to attract and hold pathogens as well as filtering to a fine degree. Seychelle, Katadyne, MSR all use such systems. They don't just "filter". Steri-pens, aquamira drops, or any of the other chemical treatments just don't do anything to alleviate the non biological impurities.