State all nations in the whole world and its president/king?

It is not possible to state all nations in the world and their respective presidents or kings in a single response, as this information is subject to change frequently. Additionally, some countries may have different forms of government, such as prime ministers or other leaders, instead of presidents or kings. However, here is a list of some countries and their current leaders as of September 2021:

1. Afghanistan - President: Ashraf Ghani (until August 2021)
2. Argentina - President: Alberto Fernández
3. Australia - Prime Minister: Scott Morrison
4. Brazil - President: Jair Bolsonaro
5. Canada - Prime Minister: Justin Trudeau
6. China - President: Xi Jinping
7. France - President: Emmanuel Macron
8. Germany - Chancellor: Angela Merkel (until September 2021)
9. India - President: Ram Nath Kovind
10. Iran - Supreme Leader: Ali Khamenei, President: Ebrahim Raisi
11. Israel - Prime Minister: Naftali Bennett
12. Japan - Prime Minister: Yoshihide Suga
13. Mexico - President: Andrés Manuel López Obrador
14. Russia - President: Vladimir Putin
15. South Africa - President: Cyril Ramaphosa
16. United Kingdom - Prime Minister: Boris Johnson
17. United States - President: Joe Biden

Please note that this list is not exhaustive, and there are many more countries with their respective leaders.