I still love my Sawyer gravity filter but the Sawyer mini is a game changer!  It's tiny, it's light, and it's fast and easy to use.  Add the price point of $24.99, and you have yourself the perfect personal filter!  Added bonus, you can make it a gravity filter with a few alterations AND, the spout will fit directly into a bite valve meaning you can fill a water bladder back up without removing it from your backpack.  That alone is the reason I use this filter so much now.  When I'm filtering water for more than a few people, I still use the next filter below.

Like many people, I started with a pump filter because they work and they've been around forever.  What I hated about them was the amount of time and effort required to filter the water.  I was spending up to 30 minutes filtering 2l of water.  Enter the gravity filters.  The Sawyer gravity filter and others like it require VERY little time and effort.  Fill the bladder, hang it from a tree, and let gravity do all the work.  With the design of the Sawyer filter, the clean end of the filter has a quick connect which snaps right into my Platypus Big Zip Hydration Reservoir.  This system allows me to filter directly into my water bladder hands free in a matter of minutes!  To keep this system clean and flowing well you simply reverse the flow to clean the filter.  This is called back flushing the system.  It is very important that you back flush this type of filter often.  As long as you do, these filters will treat literally a million gallons of water without replacing the filter.  Compared with pump filters, gravity filters are lighter, faster, and pack down to almost nothing.  These filters physically remove particles, protozoa, and bacteria down to 0.1 micron in size, including giardia, salmonella and cryptosporidia.  I especially like the handle on the dirty bag which is very useful when scooping up water.  Platypus makes a system called Gravityworks which does the exact same thing minus the convenient scoop and quick connect systems.  Other popular filters using the same technology are the Sawyer squeeze filters.  There are 3 common types of water filters which I will discuss below.  There are pros and cons to each system and depending on where you plan on using it, there is not necessarily one best option.

1.  Pump Filters essentially force water through a ceramic element often times with a carbon core.  They effectively remove bacteria and protozoa.  Pump filters with a carbon core also remove taste and odor.  These filters have been around for a long while so they have withstood the test of time.

Pros:  Effectively remove bacteria and protozoa

          Can be cleaned in the field

Cons:  Replacement filters elements are expensive.

           Much slower flow rate compared with other filters types

           Freezing can damage or destroy filter element

           Not effective against virus's.

2.  Hollow Fiber Membrane Filters are relatively new to the consumer market.  These filters use the same technology of hollow microfibers that are used in kidney dialysis.  Essentially, water travels through 1000's of hollow tubes large enough to allow water to pass but too small to allow bacteria and protozoa to pass through.  With no pumping required and no filter to replace, these filters have become extremely popular.

Pros:  Extremely fast flow rate

          No filter to replace

          Very light weight, as little as 3oz

          Effectively remove bacteria and protozoa

                                                             Cons:  Freezing can destroy filter

                                                                                       Not effective against virus's

                                                                                       Does not eliminate taste or odor unless a carbon element is combined to the system

3.  Ultraviolet or UV Filters use ultra violet light to destroy a cells ability to reproduce.  Bacteria and Virus's both make us sick by reproducing themselves in our body.  These filters are great when traveling to area's where virus's present a risk.  They rely on lithium batteries and only work well with relatively clear water.  These units will not function on very cloudy, murky, or muddy water.

Pros:  Treat virus's, bacteria, and protozoa 

          Extremely fast

          No parts subject to freezing damage

Cons:  Rely on batteries

           Does not work on cloudy or murky water

           Does not remove particulates from water without a pre-filter

           Does not remove taste or odor