I was a fairly new first officer flying a B737-200 LHR-Rome. Joanna Lumley was on board and the captain invited her to visit the flight deck. She did and stayed for landing. She was charm personified.
The late Frances Shand-Kydd, mother of the late Princess of Wales, again charming.
Michael Palin, very pleasant and unassuming gentleman.
Ted Danson, very friendly and down to earth.
We had a delayed departure out of Miami (I think) one evening on a B777 and we had a request from a Jimmy Nail who was travelling in Club who wanted a window seat when the only one left was the flight crew rest seat. We would have swapped it anyway but sent a message back that it was a deal if he would come and say hello. He did and spent 30+ minutes chatting, a very entertaining guy with feet firmly on the ground. Broad Geordie accent despite not having lived in Newcastle for many years.
The above few stand out from many and I won't mention the unpleasant ones. I have always been amazed at the ludicrous, pompous affectation of the few famous people who do not communicate directly with others. It says one thing very loudly and that is that they have a complete lack of breeding and manners.