I've met a lot of actors and Johnny Depp has to be the nicest celebrity I've encountered personally. He's not like a celebrity at all and although my time with him was very brief, he still managed to make it feel like a personal and intimate experience. He's a gem of a human being - one of the few celebrities who live up to the hype when it comes to everything said about how genuine they are.
I've also had positive experiences with Christian Bale, Kate Winslet, Keira Knightley, David Tennant, Leonardo DiCaprio, and Zachary Qunito.
Thankfully I haven't met anyone who was horrible. At worst, when I met Kylie Minogue she came across as disinterested but it was very shortly after she'd recovered from her cancer so she gets a pass, and I feel a bit crass to even mention it. It was still great to meet someone who was a childhood hero.