I always find with 'popular' celebrities it's not them themselves that are the problem but their security. I was stood next to Jessie J with a huge crowd around, she was trying to talk to us but her security practically pushed her into the awaiting vehicle. She did manage to sign a few things off a few girls and shout stuff to us all bless her. If it wasn't for her security I'm sure she'd of stood and talked to us all. Similarly with James Corden at his book signing. A lovely guy who was genuinely nice, kept asking if I was alright, and other random questions but the security were telling him he couldn't have photos and that he needed to stop talking for as long.
Nicest Celebrity I've met is Samantha Janus/Womack. I've met her 11 times now and every time she's been lovely, very warm and bubbly towards you (hugs and kisses constantly). She would happily natter to you for ages, and has done quite a few of the times I've met her. Even when she's in a rush she'll have a quick chat, pose for photos and sign anything. She's had me in stitches numerous times. Pleasant celeb to meet and when she's chatting to you you'd never think she was a celebrity.