Who is best to ask to write a reference for you: The enthusiastic post-doc who'll write the most positive reference ever, but isn't very famous, or Professor Super-Scientist, who doesn't really know you, but who is a well-known name in your field...?

the professor who is known in your field.

Getting a reference from a well known professor is good-but if it is not very specific the adcom will know you asked only because he is well known.

I'd do both. If there is a limit on references- just have them both send. They will speak to different aspects- as one doesn't know you and the other really does.

it depends on what the Prof writes, but if it's even half decent it'll be much more usefull than one from a postdoc.