It depends on whether you want a plant that flowers every year (a perennial) or just for the summer (an annual). Or would you prefer a (hardy) little shrub/plant that can stay in the planter all year round ? If you are looking for colour, are there any particular colour(s) you want ? How about growing a food plant instead (a tomato or a chilli plant, or even a few herbs - say parsley, chives etc ) all these are quite pretty to look at.
Whichever plant you decide on, it's worth buying *water retaining granules* that you mix with the compost before you put the plant into the container. These swell up and hold onto the water, slowly releasing it as required (really useful in hot, dry weather as it means you don't have to water the planter several times a day.