What is the official unit of airplane/aviation speed? Some units I read/found are commonly used in aviation: mach, knot (nautical mile per hour), mile per hour (mph), kilometer per hour (km/h or kph).

"Official" meaning like the units used by the flight data recorders, if a pilot makes a report of their speed, and so on.
Knot. Pilots think and communicate with ground personnel in knots. HUD tells you knots. Textbook tells you to increase speed to 500 knots after takeoff, and land at 150 knots.