A couple years ago, I tried a new variety of coneflower, Cheyenne Spirit and they are just beautiful, come in different colors, and best of all, make it through the winter! Another favorite is Double Delight coneflowers, a pink, shaggy-centered variety.
I love hydrangeas. The first one I had was one that was given by my company for my grandpa's funeral. I have propagated it over the years and have several now. Only downside is, they need to be practically buried in the winter. Limelights are another favorite.
Blue Flax is an unusual plant, and not necessarily hardy, but I love the shape of it and the airiness of its dainy blue flowers. The downside is, the flowers close up at midday, but it is well worth growing as an annual.