I guess it is the bright reds, but the dahila "Wheels" really attracts the butterflies, bees, and hummers. It's a beautiful flower in its own right, but the bonus of the additional beauty it attracts makes this a real plus in the garden. Add in the fact that it is a strong grower and heavy bloomer, and you have a winner all around.
The butterfly is a tiger swallowtail. Generally they are yellow with black stripes, but this is a "dark phase" and so looks a lot like a black swallowtail. In the picture below, where the sunlight is coming through the wings of the same butterfly on another dahlia, you can see the pattern.
As a point of contrast, here's what they normally look like. These are a couple of old timers, their yellows paled by age, their wings beat and tattered. Glad I can make their life a little easier, giving them a little something to eat and drink.