I remember staying in a hotel in SF where reception warned me in no uncertain terms to approach the hotel only from the front entrance and not venture to the back of the hotel and the side-street if I valued my life.
Being British, I naturally ignored this instruction and had to go to see what...
Of the places I've been to, SF wins by miles. I really love the place.
I know my worst though - Las Vegas. In fact, it's the worst city that I've ever visited, I had a week there on business and have rarely felt more miserable. My life is immeasurably better knowing that I'll never go ther again.