Top 20 strongest country in the world

By what measure? Economic or Militarily.

Economic

The Gross Domestic Product is the way economic strength is usually measured. That's the total amount of money the economy has developed. It includes almost every and any thing that involves money. So if you buy a package of gum, that's part of the GDP. The 10 largest countries in trillions of dollars are

Debt Free Nations. You want to  know this because these countries are very strong economically and can survive economic meltdowns. I have marked the ones with a (*) to show that this stat matters. (**) means the country has an extremely strong economy. East Timor may have no debt, but they also export next to nothing.

Finally what you probably asked to start with -- the military.

The problem with the military list is that it lists weaponry. The problem is not conquering a country. The problem is occupying it. Ask both Russia and the US how they feel about Afghanistan. It has broken the economies of both countries.