If you could choose only one peony to plant in a very small, sunny garden in zone 6b, and it had to have (1) strong stems to hold the flowers without any flopping, (2) flowers with a pleasant fragrance and (3) a robust, floriferous fiorm, what would it be?

I love fragrance of Little Red Gem. It is a small bush with fine foliage and single small flowers.

It is hard to find a fragrant double peony with strong stems. Maybe someone can think of a semi-double fragrant peony? Actually fragrance is a difficult trait. It depends on the environment a lot and also on a personal sense of smell.

Carol, I would say White Cap.

Probably Miss America fits all your criteria. Though if you can forego the fragrance part, I would recommend Coral Charm. This peony is such a robust and reliable bloomer whose blooms do not flop and buds not easily damaged by early freeze. It also thrives in poor soil and root competition. The only thing it needs is that it must be planted in full sun.