I can't bring myself to hate an entire country, so keeping my worst list to cities.
Calcutta - as bad as you might imagine and a bit worse. One of the few places I've felt genuinely unsafe. Guatemala City - completely dysfunctional, filthy, crowded. Adelaide - as others have noted, just really fucking dull (I spent two weeks there for work once, pity me.).
The best? That's a tough one. I've loved Brazil, Italy, India (in parts), Morocco, Guadeloupe, Turkey, Japan. I spent a week in Syria not long before the civil war broke out and fell in love with the food, architecture and very friendly people, although it was clearly very messed up even then.
Lots more places to see. Need to have this baby so I can go travelling again (with child, naturally.)