Which is the best water filter to buy

Don't know what might be best, however I currently carry a Sawyer Mini. Can't say I have used it a lot except when I consider there might be some risk, the link will show you one. Many other retailers stock them. http://www.livingsimply.co.nz/estore/category/equipment.aspx?page=1&filterby=sawyer&sortby=0 Mostly I am just careful in assessing where I drink from, in 50 plus years I have never had a problem. Many places where water is passing through areas with stock concentrations alongside or upstream of your position I would definitely filter.

There is no such thing as best, depends on how much water you use. I'm carrying a Gravity Works Platypus 2L, so that's for me and my wife. We use a fair amount of water, like for coffees, tea and cooking, and I don't like to carry much water around: https://www.rei.com/product/849793/platypus-gravityworks-filter-system-bottle-kit-2-liter So with a bit of wait I can quickly filter a lot of water. And I actually enjoy watching it. Can easily scoop up water with the big bags. If you don't drink coffee/tea, i.e. only drink water, no water for cooking, a filter that fits on top of a bottle could be useful, as you can just fill up the bottle, and drink straight from the filter.

I don't have one. I drink out of hut tanks, rivers,streams and tarns and have never been sick. I never drink from water ways before or during farm land though.

what gaiters said, we've both been tramping for decades...

Echo gaiters and wayno. For most backcountry tramping, filter not required. In NZ, catchments are generally a small area so a quick look at the relevant map is all you need. In Aus, catchments tend to be much bigger (and flow rates a lot lower) so you might find a filter gets a little more use. All that said, it does depend on where you tramp - if a filter is still on the shopping list, the Sawyer mini is very good (I have one; just doesn't get any use in NZ).

water filters are made in places like america where they are paranoid about filtering water when its not always necessary. friend did the 350km john muir trail and never used a water filter, it runs high in the mountains with amply running fresh water, the chances of contamination was minimal.. most american hikers filter by default and they get a lot of coverage in the american centric internet... I hardly hear about people getting poisoned from water in NZ. i've heard of them getting poisoned on major rivers like the wanganui, but never in the mountains unless there is a particularly dodgy old water tank that an animal has died in... but those cases are getting rarer as water tanks have been improved.