Best-Venice. You havent lived, you havent breathed until you have been to Venice.
Worst-None of them havebeen truly awful, but something in these places has made me not want to revisit them:
Rome-aside from its cultural sites, much of Rome was dirty, covered in graffiti, gypsies stealing, and some unfriendly Romans. Plus the hotel was 3* but a massive dissapointment with rude staff.
Amsterdam- We had our things stolen while we were out, they were kept inside a locker, the locker was busted open! I saw many bikes stolen on the streets whilst there. Also very cold, damp weather. Not truly awful, there were good things about Aamsterdam.
Athens (in 2001): old, dirty, congested, too hot! Transport problems, taxi drivers sticking as many people in their taxi at the same time. Meh.