As one of the easiest annual flowers to grow from seed Zinnias should be top of your list if you want to attract butterflies to your garden. While there are several perennial plants that attract butterflies Zinnias have the benefit of an extended flowering season that can last until the first frosts. This article by Amanda which I found on the American Meadows website describes the best varieties and includes tips on how to grow them.

As soon as summer hits, Zinnias start to open up and show off their bright, bold colors. And with them comes a steady stream of butterflies: Swallowtails, Monarchs, Painted Ladies, and much more. Zinnias are one of the easiest annual wildflowers to grow from seed and one of our favorite plants for attracting butterflies to any garden ? big or small.?Best of all, planting zinnias for butterflies is easy!

Butterflies are most drawn to the taller Zinnia varieties with a clear yellow center, which is the disk floret that provides nectar to pollinators. Some of the double varieties (although spectacular) hide this disk floret, and butterflies may skip over that plant because it’s harder for them to find the food as quickly.
One of the main reasons why Zinnias play such an important role in the butterfly garden ? besides their easily-accessible nectar ? is the fact that they bloom from summer all the way through frost. To help Zinnias put out new blooms all the way through fall, deadhead spent flowers as they turn brown. This keeps the plant from using its energy to go to seed, and instead inspires it to use energy to produce more flowers!

See more at American Meadows