With regard to keystone plants, goldenrod is the all time winner, supporting hundreds of species of moths, butterflies, bees, and wasps. Think of it as the "oak" of herbaceous plants. Doug Tallamy has a section in his book, "nature's best hope" just for goldenrod. You may also consider other plants for the following seasons:
Spring: violets; Columbine; bulbs, crocus (excellent for early emergence of bees)
Summer: joe Pye, milkweed, bergamot, liatris, (this list is long)
Fall: goldenrod, asters
