One of my most favorite names for a girl is Agatha, so I love the "Ag" sound. Naturally, I think that both Agnes and Agnessa are also beautiful, despite preferring Agatha.
I love Agnes so much. I think it's so woolly and warm. Agnessa is interesting, but I definitely like it less than Agnes. It feels a little overdone. Like Matildalina or Mildredette or something.