Ben Nevis is the highest mountain in the British Isles. It is in Scotland. It is 1,345 metres. 

https://en.wikipedia.org/wiki/Ben_Nevis

Scotland is in Britain. Therefore, Ben Nevis is the highest mountain in Great Britain.

Permit me to emphasise once again that Scotland is in Britain

(in case a certain user notices this thread and tries to leave comments claiming that Ben Nevis isn't a British mountain).

LOL

Jabal Shams is the highest mountain in Saltunate of Oman. It is 3100 meters.

Mount Ben Nevis is the highest mountain in the British Isles. It is 1,345 meters.

Mount Toubkal is the highest peak in Morocco, It is 4,167 meters.

Mount Pico da Neblina is the highest point in Brazil. It's 2,995 meters.

Denali is the highest mountain peak in America. It is 6190 meters.

Mount Elbrus is the highest mountain in Russia. It is 5,642 meters.

Mount Damavand is the highest peak in Iran. 5,609.2 meters.

Mount Damavand (5,609.2 m ) is the highest peak in Iran and the Middle East, and the highest volcano in Asia. I have climbed it both in winter and summer a few times. Also, I have to mention that Mount Damāvand is the 12th most prominent peak in the world, and the third most prominent in Asia after Mount Everest and Puncak Jaya. 

Here is a Wikipedia page about Damavand:

https://en.wikipedia.org/wiki/Mount_Damavand

The highest mountain in in the United States is Denali, in Alaska: 6190.5 meters or, as we say, 20,310 feet. It is, in fact, the highest mountain in the continent of North America and thus one of the "Seven Summits."

When I was a child, it was called Mount McKinley, after one of the presidents of the United States, but in 2015 it was officially renamed Denali, restoring the Native American name.

The highest elevation I have ever been at is the top of Mount Evans in Colorado, 4,350 meters or 14,271-feet; we drove our car to 13,000 feet, parked, and took an easy trail from there to the top. In the United States all of the really tall mountains are in the Rocky Mountain range in the West. I live in the East where there is geologically old mountain range, the Appalachians, which are smooth, rounded, and, mostly, covered with trees. I think the highest I have ever climbed myself is Cadillac Mountain in Maine, 466 meters or 1,530 feet.

The highest mountain in my neighborhood is Great Blue Hill, 195 meters or 635 feet. It has a certain fame because on top of it is Blue Hill Meteorological Observatory, founded in 1885, which has compiled the longest continuous weather records anywhere in the United States. For continuity, they continue to use the same instruments--barometers filled with mercury, for example. I've hiked to the top many times--it is a beautiful place to see fall foliage. I also like to claim Buck's Hill, about the same height, also in the Blue Hills, because due to a fire it has no trees on it and thus spectacular 360° views in all directions.