Which mountain is not named. Tabor, Hermon and Nebo have all been proposed. This question probably doesn’t affect the content of the “vision” (Jesus calls it an ‘orama’ in Greek) but here is case for Nebo.
Harry Whittaker Studies in the Gospels 105. Transfiguration (Matt. 17:1-13; Mark 9:2-13; Luke 9:28-36)*