For Japan it's probably a tie between "heart of a flea, brain of a shark" Mori and sinister crypto Nazi Taro Aso (the LDP prefers to keep that sort of thing at least a bit more subtle). The Democrats were all fairly incompetent, but it was largely a particularly malignant behind the scenes power player that blew their singular term in office. I don't like Koizumi or (esp) Abe but they largely made their mark.
Italy is quite obviously evil clown Berluconi, although D'Alema was a rather underrated source of horror.
In Australia it is hands down Tony Abbott. A neverending source of humiliation there.
In Canada, Paul Martin was pretty atrocious iirc.
Paul Martin was PM when SSM happened there to his credit.
