Which country on the planet has the most powerful armed forces? It's not a matter of numbers, although that's a major factor. It's more a matter of other things that are not often discussed.

By size (number of troops), the top ten nations looks like this;

China
United States
India
Korea, North
Russia
Korea, South
Pakistan
Israel
Turkey
Iran

But anyone who has studied military history knows that the number of troops is a misleading measure. There are several factors that make the troops of one army more effective than others. The most obvious modifying factor is weapons and equipment (quantity and quality). Closely related to this are the combat support elements. The most important of these are logistics (being able to move troops, and their supplies, long distances and in a timely manner) and maintenance (keeping things in repair and running under all conditions.) Then there are the intangibles (like leadership, training and the most intangible item of all; military tradition.) Apply all of those to the raw number of troops and you get different number. This number is called "combat power."

Top Ten By Combat Power

United States
China
Israel
India
Russia
Korea, South
Korea, North
United Kingdom
Turkey
Pakistan

The most unusual entry here is Israel. But this is because Israel is one of the few nations to have a reserve army that can be mobilized for action more quickly than most countries can get their active duties into shape for combat. The mobilized Israeli armed forces number over half a million troops. In addition, the Israelis have world class equipment and weapons, as well as exceptional intangibles. The downsize of this is that mobilizing its armed forces also cripples the Israeli economy. Under these conditions, Israel must conduct a war that ends within a few months. After that, supplying the armed forces becomes difficult and actual combat power begins to decline. 

The other nations in the top ten have large armed forces that are well equipped and trained, at least compared to most nations farther down on the list. Britains armed forces, like Israels, are better equipped, trained and more experienced than most. Turkey benefits from having a strong military tradition and excellent leadership at the small unit level, as well as good combat training.

Overall, the U.S. combat power is about three times that of second place China, and ten times that of tenth place Pakistan. But another modifying factor is how you plan to use that combat power. Wars are not fought in a vacuum, but in places that often inconvenient places for one side. Most armed forces are optimized for fighting on their own borders; for defending the homeland. Only the United States is capable of quickly moving lots of combat power to anywhere on the planet. Moreover, given a few months, the United States can put enough combat power just about anywhere, and become the major military force in that neighborhood. Countries like Britain and France can move some forces to just about anywhere on the planet. But no one can put forces anywhere quite like the United States.

For most nations with powerful armed forces, it's mainly a matter of having the most formidable military force in the neighborhood.