"Best" is such a highly broad term. To live? Visit a few days?
I've had a blast in everyone of these cities, but only a few would I care to be there for more than a couple weeks at a time.
NYC - Awesome
LA - Awesome
Chicago - almost awesome
Boston - I really like it .. but I don't think most Utah folks would.
Houston is maybe last for me, but it's got some cool stuff
D.C. doesn't do it for me other than a few very cool spots, but otherwise a meh.
Seattle is a way cool town
San Fran is cool but overrated, imo.
San Diego weather is awesome, otherwise predictable.
Laugh if you guys want, but Nashville is a killer town. (in a good way)