Who witnessed the Transfiguration?

The apostles Peter, James, and John witnessed this event. Pope Benedict XVI said:

The mystery of the Transfiguration must not be separated from the context of the path Jesus is following. He is now decisively oriented to fulfilling His mission, knowing all too well that to arrive at the Resurrection He must pass through the Passion and death on the Cross. He had spoken openly of this to His disciples; but they did not understand, on the contrary they rejected this prospect because they were not reasoning in accordance with God, but in accordance with men (cf. Mt 16:23).
It is for this reason that Jesus takes three of them with Him up the mountain and reveals His divine glory, the splendour of Truth and of Love. Jesus wants this light to illuminate their hearts when they pass through the thick darkness of His Passion and death, when the folly of the Cross becomes unbearable to them. God is light, and Jesus wishes to give His closest friends the experience of this light which dwells within Him.