In the 10 years or so since I got out, I still wear various boots for everything from summers in Louisiana to winters in western PA. My work and recreation take me from everything to swamps to some pretty rugged hilly trails...and offices.
Hands down, the most comfortable boots I own are the Inov-8 Roclite 400 GTX. They are marketed as the lightest, all leather Gore-Tex boot made, and I tend to believe them:
They are extremely light, weighing very little more than most running shoes. On the down side, some of the weight saving comes from the abandonment of a traditional heel cup, so there's a little less ankle rigidity, so if you've got a real heavy pack, they're probably not the optimum boot. Also, the sole is a very soft, gummy composition that gives exceptionally good traction on smooth, slick rock or concrete, but tends to wear on the faster side.
In more rugged terrain, I also have a pair of Tactical Research Khybers:
They are much lighter than they might appear at first look. They give much better ankle support than the roclites. The sole material is harder and will wear longer than the roclites, but you do sacrifice a bit of flexibility.