What mountain has the tallest Mountain face?

Answer 1

A mountain isn't perfectly round or symmetrical, but is usually divided into more-or-less identifiable "sides" separated from each others by certain distinguishing features like ridges, ravines and such. These, together with the cardinal points of the compass makes it possible to talk about a mountain's "north" face for instance.

Answer 2

It depends somewhat on what you mean by "face", if you mean steep rocky ground that you need climbing equipment to move up then Nanda Parbat in Pakistan(Rupal face) and Dhualgiri in Nepal(south west face) are pretty similar at around 4000-4200 meters.

If you mean just rising relatively steeply over surrounding terrain without any mountains/hills in the way then Rakaposhi also in Pakistan rises almost 6000 meters from the Hunza valley in a few miles.