I used to live in NYC so that's my favorite. Honestly you could live there for a hundred years and never run out of places to explore. It's got a great mix of people and professions that thrive there. Great movies, art, music, fashion, architecture, etc.
I also love London. I've only been there twice but I LOVE the look of it. Plus it's the easiest foreign city for me to navigate because I speak the language fluently
Paris is just....well, romantic. So much of it is so incredibly beautiful it makes my heart explode.
Chicago doesn't get nearly enough praise. It's on the water, it's got killer shopping (the "magnificent mile" is a must if you go there) and I also love the restuarants. I can't really say I've seen the whole city, though. Even though I've been several times, I was only ever in the nicer sections of the city. I heard horror stories about the not-so-nice sections.
Boston is also a great city. It feels very old-world for the US.