Question
I have built window boxes the entire lenght of my front porch and was wondering the best type of flowers to plant.

Answer
The best place to start, is to learn about flowers.
"Perennials" don't freeze in the winter, although they usually look ugly all winter, or die back. They come back in the spring. BUT. .. they only bloom for a short time. They often need to be divided after a time. They need to be dead headed,( remove spent flowers)and they need fertilizer since you probably won't change the soil they are in every year.
"Annuals" just last one year. There are hardy annuals that may come back, but they are not reliable. They will not take as much work as the perennials, once you get them planted. They usually bloom all summer non stop.

I know you are saying, but I don't want to plant every year, and what about the expense? Perennials cost more then annuals. Annuals previde more flower power for the buck. And planting every year gives you a chance to change your display, and try new things.
If I am planting a flower bed, I like to mix perennials and annuals. But for planting boxes I will use a beautiful, colorful, mix of annuals.
I am not sure if you have your box against a wall or on an open rail where both sides can be seen. The difference in planning a design is that, against the wall the tall plants would be in the back. When the boxes can be seen from both sides the tall plants will be in the middle. There should be draping plants to come down over the sides, as well. Limit the number of different plants to just 5 or 7 in a large box, 3 in a small box. Keep a matching pattern going in each box.
Choose colors that will go with the colors that are around the boxes.  Flowers can be a great accent to complement a color scheme of a house. If you want your porch to be relaxing, and the flowers to be soft, choose pale pinks and lavender with white.  If you want the flower box to be the focal point, and get noticed, choose red, orange, yellow, and bright blue.

Favorite plants. . . Lobelia, Alyssum, Swedish Ivy, Million bells, and Bacopa for draping plants.

Petunias, Pansies, Begonias, Impatiens, Zinnias, Geraniums, Marigolds, Dianthus, Periwinkles, Snapdragons, Dwarf Mums, And Dusty Miller for the main plants.

When you go to the nursery, choose plants that you like, then take the flats of plants and arrange them beside each other to see how they will look together. If you don't like it, it is not too late to change for something else.

Since it is so late in the season, you may want to plant winter pansies, if they are hardy in your area, and some bulbs, Then change the plants in the spring for you summer display. The bulbs can be taken out, after they bloom, and stored, then replanted in the fall.

Have fun with your new project.

bakerplanter