I have an old and close friend who's a hugely successful TV producer (comedy, mainly) with a long career, and I've met lots of famous folk at various filmings and parties he's invited me to. Can't ever recall meeting one that wasn't nice, tbh. Won't brag or bore with the list!
Most different persona on and off the set I'd say is David Walliams. Addicted to camp when there's an audience/camera, but very blokey in the green room!
Edit: I did sit on the next table to Joan Collins in Le Caprice a couple of years ago, and she appeared to be a right cow! I seem to recall she sent everything back including the water!