If I could name two things that thrill me on sight in my garden, they would be Monarch butterflies and a form of milkweed called Butterfly weed/Asclepias tuberosa.
Milkweed is named for its milky sap that gets ingested by the caterpillars. This causes predators to vomit so they soon learn to leave the yellow striped caterpillars alone.
Even if Butterfly weed was not on the Monarch menu, I would plant it. The blooms and buds make beautiful bouquets.
This plant is easy to grow and propagate. No problems with pests.
Plant some milkweed and wait for the Monarchs.