I can vouch that BZR has one of the best plain-text diff tools - whether or not it's number 1 without numbers to back the claim is a different matter though.

However, what I really want to see is BZR and GIT catching up to SVN when it comes to binary diffs - SVN remains the most efficient at finding and commiting changes to binary data between revisions. It may not seem so important, but if you're ever in a backwards country on business and need to upload to a repository via dial-up..... and of course, the smaller the binary diffs the more space saved on the server and bandwidth charges.