If you consider the highest point on Earth to be the highest measured from the Earth’s center, Ecuador’s Mount Chimborazo is your true champ.

• Tallest mountain if measure from sea level : Mount Everest.
• Tallest mountain if measure from base to top : Mauna Kea.
• Tallest mountain if measure from center of the Earth : Chimborazo

Sea level is used as a standard of measurement, because it’s the most practical. The other measurements aren’t negated in any way, but they aren’t very useful when used during climbing.