I love N.Y.C. Next to Hong Kong, and Montreal, of course, that's my favourite city that I've been to.
I like Philadeplhia as well and agree with NSW about Detroit. My least favourite U.S. city by far!
I absolutely adored New Orleans, and L.A, was pretty meh for me. The thing about L.A. is that I could never figure out when I was in the city or out. The urban sprawl is endless. I've never been to S.F. but would love to go one day because I think I would love it, and I'd get to see my 49ers in person for the first time.