I've actually been meaning to suggest this here (especially since the one on the mu forums gets spammed by the same person over and over again every few weeks) so per Andrew, here goes.
Recommend an exceptional WPMU site. Here's the kicker though: Explain why you think or feel it's an exceptional WPMu site.
I expect 500 words with each post. Standard 5 paragraph form. Neatly typed. Margins and all that.
edit: I was kidding about the 500 words.