Everywhere..and nowhere.
Even with the most lethal weapons controlling the world would be quite difficult. Look at the US invasion of Iraq. Compared to the weapons Iraq had the US had an overwhelming weapon superiority, in numbers, quality, and technology. They still ended up leaving Iraq without any real success. Why, because destroying a nations defenses are not the same as controlling the people.
To control the people you need effectively boots on the ground, intelligent people right there to control the area your in. A giant super-bomber or orbital death ray is great at blowing up big military weapons, but it does little to keep people from refusing to pay taxes, or rioting, or gurella warfare with simple weapons that are no threat to orbital death rays, but still lead your infantry to be afraid of leaving their barracks.
Unless your willing to do a full on terror mode, where you make everyone too terrified of you to do anything else by killing anyone that does anything at all to upset you your find the people resisting your rule vehemently long long after your super-weapons have destroyed all major weapons of war. Even if you go full on totalitarian your going to need a very loyal defensive guard to protect against assassins, and spend your life trapped in a bunker for fear that if you go outside so many people hating you means multiple attempts on your life.
This is assuming you have advanced but not 'god-mode' level technology. If your have controllable mass-produced strong AI, smart nanotech, or big-brother level of spying power you may be better off, but if you have such game breaking power you can pretty much rule from anywhere.
In short, military conquest of the world is not going to be a good idea. Your make a world where everyone resits you, assassins are everywhere, and either no work get's done, or work only gets done because you are the most evil of totalitarian governor everywhere that make people work via constant fear of death.
Of course there is another option, you could just take over the world economically. You have technology worlds beyond what everyone else has after all. Sell it! First create some black box, so others can't produce whatever it is you sell, then set up a perfect monopoly on technology everyone wants.
In the most benign scenario you become what legends like steve jobs and bill gates are/were, only three times as much. You have so much money you don't know what to do with it, everyone respects your genius, you can play on the big stage, and you don't really need to do anything but enjoy the easy life.
If you really must control the governments, not just enjoy being the richest and most important person in the world, then you can always use your monopoly to control them. If the government doesn't go along with what you demand your instant-food-replicators will no longer be provided to that nation, and they can go back to having their population starve while everyone eats all the free easy food you are providing. Your super-computer come with backdoors that you can use to shut them off if someone doesn't cooperate; killing their economy etc. Once your technology is mandatory you have massive control to dictate terms to those who want it.
If you do this politely and subtly enough people won't realize how much power you have, or if they do at least won't hate you enough to want to resist you because of it (again, look at the monopolies currently, Microsoft and apple have both had some practices we didn't like as consumers, both have tried to force their monopolies on the world through shade methods; but ultimately they provided the best products and we still used them even if we complained about their practices).
This seems a better way to control more of the world, with you leading a much easier life and a higher chance of actually working, and as a minor added bonus, doesn't require a war that decimates the world population (you know, minor plus) :)
