I haven't really had one ... well except for when I was doing my CPA and it was a requirement for completion. Other than that, although I have wanted one sometimes it has never worked out.
The question is:
Who's your dream mentor?
I can honestly say that I really don't know. I think it would depend on the context - are they for work or something else? Whoever they are need to be a person of integrity, knowledgeable in their field, and actually care about me.