Hummingbirds love nectar from flowers, but not all flowers are easy to feed from. They always choose food sources that provides the most nectar and the easiest nectar with the least amount of effort. That's why feeders are at the top of their list for food. They can sit and feed while using up little energy and gaining the most amount of calories.
Here's a closeup of a young Ruby Throat feeding from a rich nectared Vining Nasturtium flower. This annual produces stiff petaled flowers which are a big advantage over Petunias. When they insert their beak deep within petunia flowers, it creates a vacuum effect and pulls the soft petals towards the wings and hinders their flying. The stiff petals on Nasturtiums are rigid enough to hold their shape and not allow this problem.
I still like the Petunias, but you may want to also consider the Million Bells(Calibrachoa). They are smaller, stiffer, and rate even higher than the Petunias with hummingbirds.
N.E. of Edmonton, Alberta. August, 2015