Its had to pick just one as a favorite with so many beautiful perennials to choose from. I love Iris and wish that it had a much longer growing season than it does. It has such a sensual elegance to it, intricate and yet simplistically beautiful. Lilies of all kinds are high on my list and they do last for a long time. Lavender is another, large growths of it are spectacular. I can grow it for a season but have difficulty getting it to come back year after year and end up having to replant it.