For around $60, the Katadyn Hiker is a good filter that’s been around for over 30 years and is still considered one of the top models in terms of performance, reliability and cost. The First Need XL costs about twice the price but widely considered the best of all portable hand-pumped water filters. Several units by MSR are lightweight, compact and effective against protozoa, bacteria, viruses AND chemical contamination.

I have also used gravity filter systems from MSR and while I initially found them convenient and effective, (fill and hang before setting-up camp, water’s ready by the time you’re done), I found that over the long-term they were less convenient because of the frequent back-flushing necessary to maintain adequate flow.

Do your on-line research on sites such as REI then visit your local outfitter or outdoors/camping retailer to compare size/weight and get additional recommendations.

You do need to understand that there is always the potential for any water filter to fail on the trail. You should carry at least one backup method of purifying water. I also bring chlorine-dioxide (unscented household bleach) in a dropper-bottle, and have a metal cup or a cook-set with a stove and fuel to boil as a last resort.

Note that with boiling it is not necessary to maintain a boil for long periods. Clinical research indicates that merely bringing water to a rolling boil is sufficient to kill all pathogens (though at higher altitudes the boiling point occurs at lower temperatures, so above 8,000 feet ASL you should boil for 3 minutes).

Iodine has proven to be unreliable at destroying Cryptosporidium, a common waterborne protozoa, and often is not well tolerated (taste), resulting in higher risks for dehydration. Chlorine Dioxide (tablets or simple household bleach at a rate of 3 to 5 drops per liter) will kill all pathogens but requires up to 4 hours treatment time to reliably destroy Crypto. As mentioned above, surface contamination is a problem with chemical treatment, so either ensure the lip and treads of your water container are exposed to the disinfecting chemicals or transfer disinfected water to a known clean container before drinking.

The following link is a short but helpful article from Family Physicians Inquiry Network on water purification methods.
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References :
http://www.jfponline.com/pdf%2F5701%2F5701JFP_ClinicalInquiries1.pdf

You can buy a small bottle of 2 % Iodine U.S.P. or 2% Iodine B.P. at the store for about $2-$3. You mix 2 drops, like from an Eye Dropper to 1 quart of water (the actual ratio is 2 drops for 1 liter, but here in the states and with U.S. Equipment Quarts are easier), allow to sit for 30 minutes in bottle capped (When I use this method I use double the Iodine so 4 drops, taste worse but is safer, you can eyeball it, it’s safe to have a little to much, but not safe to have too little, leave the cap alittle loose turn upside down getting the solution on the threads or lip of your drinking utensil so it is treated also where you are going to have your mouth, "you shouldn’t put your mouth on your canteen or what not anyway). This is going to taste bad, in the Army they used to have us mix floride in for taste, and to clear the Iodine color from the water, so put in some tooth paste, about a fingernail size amount, cap and shake (you can drink now, but will take 15 minutes to clear the color from the Iodine out of the water. Don’t drink it fast as it can give a mild headache. Until you get used to it. It’s potable and safe however.

If you wanted to make a large amount of water, you can use actual Plain bleach, like clorox (unscented), not sure on the ratio, anymore, this is not Ideal for Hiking. It alos tastes and smells awful
and the Chlorine (which is what your using out of the Bleach) delutes rather fast, compared to Iodines stable shelf life of about 3 years.

A can of sterno, and a pot, boil for 5 minutes (at sea level, add 1 minute for every 1,000 ft above)

**********I got a thumbs down, or a boooo! as I call them, so I have a website, that I just googled about the Iodine thing, they may be a little over cautious on the site but it’s the same info, as I learned in the Army, they go stronger than we were taught, I always eyeball it useing the cap to measure:

http://www.instructables.com/id/How-To-Purify-Water-Using-Iodine-Tincture/

http://msucares.com/disaster/faq3.html

This website states that the Iodine treatment can also attack Bacteria and Viruses that may have made it into your body, which I find hard to believe since it goes inactive the iodine goes inactive, Iodine is an actual organism, like, but not the same as rust, on this site they also do pro’s and con’s to the different methods, it’s sources are quoted as the CDC, the U.S. Army, and other big ones:

http://zenbackpacking.net/WaterFilterPurifierTreatment.htm

Also a filtration system requires clean filters, once the filter goes out your water may be contaminated without your knowledge, with Chemical purification it’s clean and you know it, it’s reliable. Some filtration systems require batteries and any thing electrical can fail for any number of reasons at a moments notice. It’s cheap and it’s light, and continues purifieng in your body with every gulp (according to the last website), one of these sites suggested adding Vitamin C for flavor instead of floride. Each site suggests between 5-10 drops per Quart (over kill), but I have had Giardia before, I had no water treatment available at the moment, and I drank any way and got it, It is treatable but sucks, you are given a drug called Flexeril for a week and it’s as bad if not worse than the parasites.

With Iodine you don’t have to carry around the extra water weight, you can collect as needed. I know it works. I keep some at home, about 2 years ago in downtown Portland, Oregon (where I am), an animal crawled into the cities water resevoir and died, there was an outbreak of Ecoli, which was announced through the Emergency Bradcast System, I did the Iodine thing and Boiled my water, in that case you know for a fact there is an outbreak, and it lasted for 3 days.

OH, yeah, the longer you let it sit the better it tastes, it settles or something. Sorry for my Grammer I was Tired when I wrote most of this, English is my Third Language, Dutch was my First, and Gaelic was my Second. Not to mention this is a serious topic for the asker and people that use the Information on this web page, so it is good to be accurate, could mean life or death.
References :
36 Year old ordained Irish/Dutch/Jewish French taoist Disabled Veteran (1993-1997). Former U.S. Army Paratrooper, 1/508th P.I.R. in Panama, HHC/75th Ranger Regmnt., 2/325th A.I.R. 82nd Airborne Division, 18th Airborne Corps Test Board, Army Spec Ops,, Jungle School Op For, Darby Phase Op For, U.S.A.R.S.O.

I have a "first need" been using it for well over ten years now with no issues.
http://www.rei.com/product/767831/first-need-xl-water-purifier
References :