Butterflies on ... Phlox paniculata
This is John Fanick's phlox, a variety of the old fashioned 'summer phlox' or Phlox paniculata. Phlox of all kinds are among my very favorite plants for summer color and as a butterfly nectar flower. It's a nicely scented flower that's great for bouquets also.
Tiger swallowtails seem especially attracted to it.