NYC is THE city.

Hey Flyer,

Relax. You made your point the first time. There are others who agree and who disagree. What's the problem?

mm

P.S. "What's the problem?" is not a solicitation for another response.

San Francisco. I think the city embodies beauty, culture, and a care free attitude. I can't comment on NYC, because I haven't yet visited. It is definately an icon. Las Vegas?? Of all of the beauty in our country, have someone have their first taste be the ultimate city of knock offs? No way!

Aside from the fact that I've always far preferred nature to people, I would pick unique and wonderful San Francisco.

Whoa there, AA, nowhere did the OP use the word "unique." He/she just asked for our opinions on the "ONE place/city" we would recommend to touristy friends coming over from Europe.
I'm sorry, but I certainly wouldn't list Las Vegas as the ONE place in the US to visit.
At the same time, I'd be very hard pressed to name that ONE place, since ours is such a diverse country is all ways.
If I could pick a few, my very personal list would include at least (in no particular order):
New York City
Oregon coast
Grand Teton National Park
Monument Valley at sunrise