Petunias, snapdragons, dianthus, verbena, portulaca, geraniums... pretty much anything still available at garden centers, etc. would still have time to settle in and put on a good show until fall. Annuals are pretty well selling out up here by this time, though. The big box stores and some of the garden centers don't seem to be doing a lot of annuals restocking once the big push is over.

I think geraniums would probably be best. Because the flower heads are so large, there's a good impact with each plant.

It's already July, and if you buy flats of things like petunias, portulaca, marigolds, etc. they just aren't going to fill in much by the end of the season.

If you don't want geranium or they wouldn't do well in that spot, whatever you get, get the larger pots (at least the super 6-pack size, even better if can get 4" or 6" pots) - otherwise, you'll probably have a disappointing display.

Get some Wave or Tidal Wave petunais. I just found a bunch at 50% off. Give em good soil water sun and fertizer and they will take off! Since they tend to get scraggly at the stores you might want to cut them back a bit to promote new growth. Also you might want to tease the roots out a bit as they will be very root bound by now.