Vic & Bob. Vic was a bit 'aloof' (still charming, just a bit shy, which I didn't expect!) but Bob was a grade-A LEGEND! 
Derren Brown- was super nice; I had a long discussion with him about cold reading and he recommended me a few books on the subject, I've still got the list he wrote down somewhere. Very lovely chap.
Literally bumped into Nigel Havers coming out of a chippy in London once. I think I stood on his foot. He was very gracious about it all.
Cerys Matthews- hard work. This was in her early drink/drug Cardiff days. She seems to have mellowed out a bit now! I know of at least 2 establishments she was barred from. At one event I was working she was warned a number of times by security staff for standing/dancing on tables and got terribly gobby when challenged. We've all been there, I s'pose!
Manic Street Preachers- very self-effacing and down to earth. Nicky Wire isn't as tall as you think he is; the others are just all tiny. Worked an event where they were at least 80% more famous than everyone else there, but you wouldn't have known it by their general aura of coolness.
I also got 'chatted up' by (SIR) Anthony Hopkins whilst I was working behind the bar at an event!
He's not my cup of tea but actually very charming, for an older fellow.
Fortunately, I've never met any 'nasty' celebs- sorry to be boring!