There are literally hundreds of options you can choose from. I can name my favorites, or you can scroll down to the alphabet, click letter 'A', annuals and sort through all the different choices until something catches you fancy.

Lavatera is a favourite of mine and there is now a shorter variety only a foot tall. If you have poor soil the dwarf strain of nasturtium is showy all summer but no good in good soil as the leaves swamp the flowers.
Annual cornflowers good in borders, not so much for pots.
Annual Osteospermum. Amaranthus for its long tassels.
Gazanias if you like them. Larkspur. Tagetes for front of border.