What plants are best to have for hosting and attracting butterflies?
A large number of plant species are used by one or more species of butterfly. Which plants are best depends on what type of butterflies you want to attract and the environment/conditions in which you want to grow the plant.
Here are some FAQ entries from the Butterfly Forum:
What plants are good nectar sources for butterflies?
What host plants are grown to feed caterpillars?
You may want to check out the other FAQ entries in the Butterfly Forum, as well.
You could also google things like 'butterfly host plants' or 'butterfly nectar sources' for more information.