What's unforgivable?

I'm working on an idea for a message series and I'd like your thoughts. The base of the series would be launched from a message entitled "The Unforgivable Sin". Here's how you can help, what do you think the unforgivable sin is? Or, what sins do you think God refuses to forgive? Is there one, two, or none? Hope you post something, because I'd really like your thoughts! This could be my first internet collaborative message series!