Some people might say it is ‘Kanchenjunga,’ which is also situated in the Himalayas, or some will say it is ‘Mauna Kea’ in Hawaii. Kanchenjunga was considered the tallest mountain before the discovery of Everest in 1852, and Mauna Kea is the tallest mountain from the base to peak. But Everest has the highest elevation.
Even though Everest was discovered later, it had the highest elevation all along. So the answer is ‘Everest’ itself.