1,268 kilometres/hour, which is 787 miles/hour on a SQ flight from SIN to SYD many years ago with a ~300km/hour tailwind. The speed was reported by both the inflight system and my GPS (with a 2km/hour difference between the two)

Yes, that's technically faster than the speed of sound, but as it's relative to the air around the plane we weren't actually above the sound limit (obviously!)

We landed in SYD well before curfew, were met by an ambulance to handle the medical emergency which was the reason for the speed, and then had to wait for close to an hour before immigration arrived and we could deplane.