The best boot is the one that fits your foot, keeps them warm, and has a liner than can be dried out easily. There are numerous models that fit those criteria.
As stated before, Millet and Sportiva Olympus Mons are front runners in the integrated double boot category, meaning you don't have to have an overboot with these. Otherwise you can go for a more versitile boot like the Sportiva Nuptse or a plastic boot with an Intuition liner and combine that with an overboot. That system gives you more flexibility when it comes to using your boots in places other than high altitude slogging, not to mention more technical performance compared to something like the Olympus Mons.