Which city?

The United States is a very biiiiiig place. You can't expect any useful suggestions w/oat least SOME details. Where are you coming from? What sorts of things do you want to see/do?

How warm do you want to be?
What kind of atmosphere (Rome and London are both in Europe)?
Do you want a direct flight or are you willing to take a second flight within the US?
Do you want to lie by a pool, hike a trail, visit museums?

If not a troll a big if post up some details but
I might leave my heart in San Francisco VERY romantic.

As noted - it also partially depends on where you are flying from, and what you want to do. NYC - or the beach - or mountains? And do you want to spend it all in a city - or partly in a city and then getting outside of a city to see some beautiful sights of nature?

Agree - without more info it's imossible to give a reco.

Most places have pleasant weather in May.

but do you want a beach, a big city with museums, theater and fine dining, the mountains, natural wonders?

How long do you have?

Where are you coming from?

WOW that is one HUGE question. It's really impossible to answer without knowing what you are looking for. Since you did mention "city" (as opposed to smaller towns or the countryside) I'll toss out...

San Francisco
Los Angeles
New Orleans
Miami
New York
Boston

Will you be willing to rent a car or not? Some cities have good public transports, others do not (in LA you would spend lots of time taking buses, etc. and some places you just cannot get there from here).

Honolulu.

Thanks to all answers.
Long time here again, sorry.
I want a beach, a city with fine dining and the mountains.
Two weeks there.
Thanks again.