i've only lived in a few american cities: new york city, hanover new hampshire, and claremont california. i have to say of the 3, new york was the friendliest.
when i first moved to the city (i was fortunate to find a cheap studio in the chelsea/meatpacking district in manhattan), i bought a coffee table from ikea which i thought i could 'carry' back to my apartment. i didn't have a car so i took the subway. the station was only a few blocks from my studio and the new coffee table was a challenge to carry back, but i was determined. as i got out of the station, an elderly woman going in the opposite direction had a small cart, with wheels that looked as if they were going to break any minute. she looked at me struggling, stopped for a second, then offered to help push my purchase back to my apartment. i was a little apprehensive as i didn't want to break her cart, but she was insistent. as we arrived to the front stairs, a man walking by offered to carry it up to my fourth floor studio. i wasn't expecting or even looking for help, but it was offered willingly. this was 6 years ago, but this impression of new yorkers has stuck with me. so new york gets my biased vote as the friendliest city
i must say, tho, the cold weather can damper ones mood! especially when the street grid creates a suction for wind to blow at incredible speeds :-/ but all in all, i think this says more about human nature and individuals empathy towards others than the city per se.