What's ideal in central Europe is the "Buddleja davidii".
Buddleja davidii (spelling variant Buddleia davidii), also called
summer lilac, butterfly-bush, or orange eye, is a species of flowering
plant in the family Scrophulariaceae, native to Sichuan and Hubei
provinces in central China, and also Japan.1 It is widely used as an
ornamental plant, and many named varieties are in cultivation.
They are an ideal nectar source for many butterflies. As already stated it's even called the "butterfly-bush".
Buddleja davidii cultivars are much appreciated worldwide as
ornamentals and for the value of their flowers as a nectar source for
many species of butterfly, though the species and cultivars are not
able to survive the harsh winters of northern or montane climates,
being killed by temperatures below about −15 to −20 °C (5 to −4 °F).
However, I don't know if it's suitable for your region. I believe (!) that it's a very uncomplicated plant due to the fact that it was imported from China long time ago.
Butterfly Garden Collection
There's a great flower collection called "Ann and O.J. Weber Butterfly Garden". It states nearly all suitable plants to attract more butterflies. It's also possible to narrow the search (left side in the page) to filter it by region, height etc.
You may also consider to use some "host plants" like described on "Gardens With Wings".
By including both host plants and nectar plants in your garden, you
can attract a wider selection of butterflies while providing an
environment that supports their entire life cycle.
List of host plants:
Flowers: Aster (Aster spp.) Black-Eyed Susan (Rudbeckia hirta)
Butterfly Milkweed (Asclepias tuberosa) Common Milkweed (Asclepias
syriaca) Coneflowers (Echinacea purpurea) False Nettle (Boehmeria
cylindrica) Hollyhock (Alcea rosea) Indian Paintbrush (Castilleja
spp.) Mallow (Malva spp.) Nasturtium (Tropaeolum majus) Pussy-toe
(Antennaria plantaginifolia) Rue (Ruta graveolens) Ruellia (Ruellia
spp.) Shasta Daisy (Leucanthemum spp.) Silver Brocade (Artemisia
stellariana) Snapdragon (Antirrhinum majus) Spider flower (Cleome
hasslerana) Sunflower (Helianthus spp.) Swamp Milkweed (Asclepias
incarnata) Swamp Verbena (Verbena hastata) Tall Verbena (Verbena
bonariensis) Violet (Viola spp. ) Water Dock (Rumex verticillatus)
Wild Senna (Senna hebecarpa) Woodland Stonecrop (Sedum ternatum)
Herbs: Dill (Antheum graveolens) Fennel (Foeniculum vulgare) Parsley
Grasses: Little Bluestem Grass (Schizachyrium scoparium) Orchard Grass
(Dactylis glomerata ) Panic Grass (Panicum spp.)
Shrubs: Coontie (Zamia pumila) False Indigo (Baptisia australis)
Spicebush (Lindera benzoin )
Vines: Passion Flowers (Passiflora spp.) Pipevine (Aristolochia
Trees: Aspen Tree (Populus spp.) Common HopTree (Ptelea trifoliata)
Elm Tree (Ulmus spp. ) Flowering Dogwood (Cornus) Pawpaw (Asimina
triloba) Prickly Ash (Zanthoxylum americanum) Sassafras (Sassafras
albidum) Sweet Bay (Magnolia virginiana) Willow (Salix)