I think of my annuals as the real work horses in the garden. In May I start searching for old favorites and if I'm lucky find something new and striking. It's usually possible to find some already blooming plants so there won't be any surprises in color. I'm lucky to have a greenhouse near me with a really good selection. I'm not fond of bare ground around my perennial plants so I tend to stick an annual anywhere there is a space in my perennial beds. I try to select annuals that aren't bothered by insects or disease. The ones that go into the ground have to be drought tolerant.
Below, heavenly scented heliotrope is one of my favorites. It looks good with any yellow blooming plants. Pictured here with old fashioned marguerite daisies.
Same thing here with some purple petunias and of course Thumper.
I like these creamy yellow petunias around the base of yellow or orange roses. Roses have gorgeous flowers but the shrub itself is ugly. Anything bushy and blooming that covers the straggly rose canes is a plus.
Below petunias with Autumn Sunset rose.
My favorite petunias are these two- toned purples. They are great with yellow blooming flowers and silvery foliaged plants. There is some dusty miller planted among these and some Achillea '
Moonshine'. 
Cleome below. Plant it once and you will have it forever. In the spring the seedlings come up thick as moss on the ground. It gets tall so I put it behind medium height plants. I like it at the base of shrubs like viburnum to add some color long after the vibernum has bloomed. It grows up through the shrub branches and looks interesting. I tried the smaller cleome 'firecracker' in pots but wasn't happy with it. (Warning, pulling up all those extra seedlings can be annoying.) Cleome is quite drought tolerant.
Rudbeckia hirta is an annual for me. Every now and then one will return or reseed, not often.
Liatris burning their lovely candles quickly and leaving a burnt stem. These vertical plants don't take up much space and can be tucked into voids around the garden. I need to remember to pick up another package of bulbs, maybe white this time.
