Miami at the Sherry seems to be a favorite for int'l crews.
Boston's hard to beat for a layover.
NYC & LAX kinda depend on how long and where's the hotel.
I'll second the votes for Anchorage.

These probably don't see the international crews much, but my personal favorite U.S. cities:
Austin is a great time.
Burlington in the summer.
Savannah.
Lots in the Pacific NW: Boise, Spokane, Missoula, Bozeman, Helena, Kalispell.
I'll add a vote for New Orleans, too.

Finally, my all-time favorite layovers in North America aren't in the states: Vancouver & San Juan!

Oh man, I've had some great times at that place...