Milkweed, asters, verbena, many. Is there any particular look you are going for? You can tell good butterfly flowers often by just looking at them. I wrote something that helps you tell a butterfly plants by their flowers, too, linked below.

Here is a link that might be useful: A guide to identifying potential butterfly plants at a glance

Annuals:
Mexican Sunflower (Tithonia)
Tropical Milkweed (Asclepias curassavica)-host for Monarchs
Blue Vervain (Verbena bonariensis)
Zinnias - DON'T get the double kind; I recommend Cut and Come Again & also California Giants, although others on here have mentioned others.

Butterfly Bush (Buddleia davidii)
Purple Coneflower (Echinacea purpurea)
Swamp Milkweed (Asclepias incarnata) - host for Monarchs
Butterfly Weed (Asclepias tuberosa) - host for Monarchs
Mallow, hollyhock - pretty flowers and host plants for Painted Lady and Common Checkered Skipper
Rue - host plant for Giant Swallowtails & Black Swallowtails
I think the following could be annual or perennial depending on your location:
Parsley, Dill, Fennel - if you want host plants for Black Swallowtail caterpillars

Those are all good recommendations - check the FYI section for more.
Sherry

Hi.. and welcome fellow CT person! You'll love it here.

My best flowers for attracting butterflies are

Zinnias, most kinds work for me, even the doubles like Benary Giants, Oklahoma, cut and come again, and California giants. Last year I found that I even had success with the shorter Profusion varietites. They attracted mostly skippers and American Ladies but as the season turned to Fall, other species checked them out. I'd never noticed the shorter types attracting much until lat year.

Of course a butterfly bush would be good, they don't cost much if you get a small one gallon pot at a local nursery.

Verbena Bonariensis works great here and most years reseeds.

Tropical Milkweed which you can grow from seed yourself is great for Monarchs.

Mexican Sunflowers (tithonia) attract Tiger Swallowtails.

I always grow parsley and bronze fennel for the Black Swallowtails.

I hope your garden is in a sunny spot, because the butterflies love the warmth of the sun.

Good Luck and keep posting. Our season up here starts late and peaks in July and August. But september and october can be rewarding months for the migrant species.

Please consider joining the Connecticut Butterfly Association..Dues are only 15 dollars and we have field trips, educational programs, and a email network that you can join where we can post our sightings.

Maryann in SE CT.

lantana, cosmos, and pentas attract alot of butterflies here, as well as others mentioned also