The largest mountain in the world is

Mount Everest, located in the Himalayas, is the highest mountain in the world, standing at a height of 29,032 feet (8,848 meters) above sea level. It is considered the tallest peak on Earth.

Note that does not count mountains that start at the bottom of the ocean but only applies to those starting above sea level.

If we consider mountains that start above sea level, the largest mountain in the world is Mauna Kea in Hawaii. While its height above sea level is 13,803 feet (4,207 meters), its total height from base to summit is around 33,476 feet (10,204 meters), making it taller than Mount Everest when measured from its base on the ocean floor.

you mean if we consider those that start BELOW sea level :)

Apologies for the confusion. If we consider mountains that start below sea level, the largest mountain in the world is actually Mauna Loa, also located in Hawaii. While its height above sea level is 13,678 feet (4,169 meters), its total height from base to summit is estimated to be about 30,085 feet (9,170 meters) when measured from the ocean floor.

Mount Everest.