I believe it was the first commercial flight into the current airport. Other aircraft were ferried empty from Kai Tak to CLK before "Polar One" landed.
Bill Clinton visited HKG shortly before CLK opened and Air Force one and the C-5 Galaxy(ies) actually used it before it was officially open.
Obviously there's also the GFS and all calibration jets which landed here as well. I believe the very first aircraft was a GFS Jetstream41.