I'm certainly no boot expert here and will give way to most here now but have come to the conclusion, that for me, heavy full-grain leather uppers -- true, stiff and heavy --with a heavy Vibram sole work best. Which I think points to the fact that ultimately individual choice may vary a bit even among knowledgeable trekkers. While I'm not a serious mountain hunter meaning at least an every-year ten day hunt above timber line I have elk hunted in the high places enough in most western states to have seen flimsy synthetic panels wear through on hard scrabble and have glued soles part company with me. I have found insulated boots (not pacs) are something my feet don't need, especially with KOM gaiters. My latest and last pair are Alico Guides at one size above my shoe size; I wear a syn liner and a medium wool sock and add a good extra insole.
These go against my usual aversion to extra weight because they are heavy and some may call them "clunky" but my feet and especially my ankles are happy in them. Prep them with Obenaufs (as I do some of our leather furniture) and they work for me.
FWIW..
