A Lesson in Aesthetics
Aesthetically, my favorite Apple machine ever is the iMac G4. My second favorite is likely a tie between the 12" PowerBook G4, and dare I say; the newly redesigned October 2009 MacBook.
The Snow iMac G3, iMac G5, iBook G3, eMac G4, and the newly redesigned October 2009 27" iMac all get an honorable mention.
Having said that, my least favorite Apple machine in the history of Apple in terms of aesthetics and display usability is the Mid-2007 iMac (with the "glass" display coating) which was recently discontinued (October 2009) with the release of the new iMac.
The most recent line of MacBook Pros (also with a heavy emphisis on black, but with excellent build quality), the MacBook Air, and the First Generation MacBook all get a not-so-honorable mention.