The loveliest celebrity I've ever met is Nerina Pallot.
(For those who don't know, she's a superb British singer-songwriter and has written for the new Kylie Minogue and Diana Vickers albums.)
Met her after one of her recent shows - incredibly friendly, chatty, down-to-earth and approachable. Not starry at all and pretty self-deprecating. Really appreciates the fans and it wasn't like a quick "hello, thanks for coming," she actually has proper conversations with her fans, and is really interested and talkative.