There is no such thing as the best multifunction printer for the Mac. What one user finds to work perfectly for him, another user may have no end of problems and curse the model and brand to the ends of the Earth.
I would suggest that you need to set your criteria first, i.e. it needs to be laser or ink based, it needs to support faxing (not all MFD's do), what price you are prepared to pay. Then other users can reply with what they have and tell you their preference.
If you were happy to stick with Canon and wanted a laser model, then I can recommend the MF4380dn. It can print, scan and fax with Mac OS X 10.6 and earlier and provides USB and Ethernet network support.
If you want a ink based model, then I have been more than happy with my MP980. It supports USB, Ethernet and wireless. It can print and scan on Mac OS X 10.6, but it has no fax option. Note that this model has been replaced by the MP990.
