As mentioned above, a boot that fits well to your foot is crucial, much more important than brands or anything else, really.
I have found that La Sportiva boots fit my feet well, so I have been using them for years.
I like to wear stiff boots when sheep hunting. I have flat feet, so I wear orthotics, and stiff boots help tremendously.
I have La Sportiva Trango's for most hunting and hiking until deep snow flies. I don't typically need insulation until it is full-blown winter, but my feet run hot so to each their own.
In late season, I have La Sportiva Nepal's, which are a stiff, insulated, mountaineering/ice climbing boot. They are awesome. My cousin that lives in AK uses these on dall/mountain goat hunts. He also has flatter feet, and the stiffness seems to help him out a ton as well.
Many of the sheep hunters I hunt with have used Mendl's or Zamberlan leather boots for decades. Stiffness seems to be the common theme amongst most of the boots I have seen guys wear sheep hunting.