If you could choose to go to one beach in the continental US, where would it be?

If you are talking about basically NOW or in the next couple of weeks, AND you want to do actual beach stuff like walking or laying on the sand in swimsuits, or even getting IN the water, then the only answer is extreme South Florida-- Miami Beach, Sanibel/Captiva, or somewhere in the Florida Keys, which is not known for having miles and miles of beaches, but DOES have enough to suffice for a getaway and will probably have the warmest temps.

Temps anywhere along Florida's northern gulf coast (Seaside, Destin, Pensacola, Ft. Walton, Panama City, etc.) and its northern Atlantic coast (Ponte Vedra Beach, St. Aug., Daytona, etc.) are far too cold to have the "warm weather beach experience". Areas around Tampa Bay are likely too cold as well.

Now, if you just want to take a walk on a beach in a sweater and shoes and look at the ocean, everything from the Pacific Coast up to around Cape Hatteras on the Atlantic is in play... I was on the beach at Hilton Head and then Jekyll Island, GA during the week between Christmas and New Year's, and that's about all it was good for.