Nearly every customer that I have ever dealt with over the years asks about the treatment on their newly installed fence and upon hearing that it has a treatment seem to promptly forget what I have suggested...

In the grand scheme this keeps fencing contractors in work so I don't want to knock it to much. 
Fencing and decking timbers are generally all from from sustainable sources and meet relevant accreditation. The reality is that these sources are fast growing pine / spruce or similar and they are not properly seasoned prior to machining into posts and rails. They are 'big grained' and hold a lot of moisture and natural oils which start to dry when they are milled and when cut very thin for fence panels or feather edge boarding dry even quicker.

Whilst posts will rot due to wet ground contact lots of other problems are due to UV drying the wood and it cracks, absorbs water, expands, drys, shrinks, cracks again etc etc.

To improve the life span of this timber I strongly recommend the annual application of a decent oil or spirit based wood preserver, I make this point because most water based treatments simply do not replace enough oil into the grain of the timber. It's my own view, but many water based fence treatments simply do no more than change the colour of wood without offering any real protection, thus I do not believe they offer any real value for the time and effort involved.

I don't have any real favourite for this but Cuprinol, Barretine, Sadolin, and Osmo make very good products.  Also don't forget Creocote. This is the new formula Creosote with some of the nasties taken out.  Always read the label on this stuff though to make sure it is suitable for your particular circumstances - child or pet friendly etc etc.