Which nation was the most powerful in the 12th century?

The Byzantine (Eastern Roman) Empire was the most powerful nation in Europe, but was in decline under pressure from the Islamic states.
In Western Asia the Seljik Sultanate (Persia) was most dominant, while in East Asia the Song Dynasty of China had hegemony.
In Africa the most powerful state was the Fatimid Caliphate in Egypt, while the Kanem Empire was the largest state in sub-Saharan Africa.
In North America the largest civilization was the Pueblo culture, further south was the Toltec Culture. Also the Iroquois Confederacy formed during this period, and would come to dominate the Great Lakes region.
In South America the Chimu culture controlled much of what is now Peru.

Many of the nations that exist today did not exist 800 years ago. In fact, few "nations" existed at all at that time.

the Mongols bar none culminating in Genghis Khan 1218 want to argue Google guys

England and France dominated.

In Europe it was the Roman Catholic Church. At one time the Church had the biggest army in all of Europe. Even without it, the Church and the threat of excommunication kept many kings in line with what the Church wanted to do. As far as out side Europe, China except it was pretty much a stay at home nation.