Well, it depends on your definition of a very hot summer, and it depends on how much rain you're getting or how much you're able to irrigate. Also it depends on whether the plants are in full sun from sunrise to sunset or if they have shade for part of the day. More than anything else, the quality of your soil matters. In good soil, flowering plants can tolerate quite a lot of heat even in full sun as long as they are getting adequate water, whether rainfall or irrigation.
I'm in south-central OK and we tend to be both hotter and drier than central OK most years, including this year. Many of my flowers grow in well-amended clay, and are mulched with 2-4" of cypress mulch throughout the growing season. I do judge the performance of all plants by how well they did, or did not, perform in the summer of 2011, when we had less than 12" of rain from January through the end of July and we were over 100 degrees at least 76 days that year, including every single day in July (a month in which zero rain fell). Anything that survived or mostly survived 2011 is bulletproof on my list. Keep in mind that even the native cacti growing on our land where they had grown for decades died in 2011, so you wouldn't expect annual flowers to outperform any perennial natives, but a couple did. Based on 2011, these are the bulletproof annual flowers that can withstand the most heat in full sun as long as they receive at least a little irrigation when no rain is falling:
Globe Amaranth, aka as gomphrena "Strawberry Fields". (Gomphrena globosa.)This grows in unimproved clay on the edge of our gravel driveway about 300' from the house and I watered it maybe once a month in the summer of 2011.
Moss Rose. (Portulaca grandiflora). This is a succulent annual that tends to reseed for me and I grew one of the standard old varieties sold in flats in stores that year, not one of the newer and pricier hybrid varieties with substantially larger flowers. At times, the succulent stems shriveled and dried up a bit (highs were in the 110-116 range during that time frame) but being watered occasionally revived them and plumped them up again.
Tall Verbena, aka verbena-on-a-stick. (Verbena bonariensis.) These survived both in the garden and in the pasture where they have reseeded. They are butterfly magnets. They reseed vigorously, and some of them may be performing as half-hardy annuals in my garden. A very wet rainy winter season is more likely to wipe them out in my garden than a very dry hot summer.
Marigolds. (Tagetes spp.) The ones the produce the best for me and that survive heat and drought the longest are the small flowered ones. I grow them every year, always have and always will. They are more prone to spider mites in hot, dry years but I usually just blast them off the undersides of the leaves with a water hose.
Laura Bush Petunias. (Petunia x violacea). This is one specific variety of petunia native to South America, and it has been bred for heat-tolerance and disease resistance, not for large flowers. With smaller flowers than the standard hybrid petunias sold in stores, this is one tough plant. The hotter it gets, the more it blooms. The leaves do not yellow like the leaves of hybrid petunia varieties. I cut back the plants by at least 50% each month in June, July and August to keep the plants from getting too leggy and taking over everything around them. Mine have reseeded regularly since the early 2000s, and you never know where they'll pop up. In 2011, they did struggle quite a bit in the heat after I stopped watering them in July, but after rain returned in mid-August, they not only revived well and put on new growth and bloomed, but new ones popped up out of the soil from seeds that apparently matured on the plants during the earlier summer months. Seed is available from Wildseed Farms.
I don't know if these two would be annuals, half-hardy annuals or perennials for you, but they perform either as half-hardy annuals or reseeding annuals for me:
Texas Hummingbird Sage (Salvia coccinea). Seed from Wildseed Farms. These are the plain native red salvias--not bred for bigger flowers or other colors or whatever. Tough as nails. Need to be watered once or twice a month or their leaves will turn black and crispy. Reseed vigorously.
Angel's Trumpets (aka Devil's Trumpets, depending on the geographic region). I grow both Datura metal and Datura inoxia. Tough as nails. Will tolerate all kinds of heat as long as they get some water occasionally. Reseed rampantly if you don't deadhead them consistently.
So, the above are the toughest. They survive the roughest conditions in the worst years. This list is the best of the rest:
Zinnias---the older varieties tolerate heat better than the newer ones with fancier flowers. Profusion zinnia and Mexican zinnia Zinnia haageana).
Lantana. These can be annuals or perennials depending on the variety. Can tolerate extreme heat with irrigation, though sometimes the leaves roll or turn crispy. Good rainfall or a good deep watering will revive them. The toughest one I grow is Texas Lantana (Lantana horrida), which has yellow and orange flowers. I have a friend whose Mrs. Huff lantana survived 2011 in very sandy soil with a lot of watering.
Mexican Hat. (Ratibida columnifera). A native wildflower that does not survive in the pastures in very hot, dry summers, but does survive in the garden if watered once or twice a month. Reseeding annuals for me.
Purple coneflowers (Echinacea purpurea). Perennials. I have both the species and a couple of the newer purple hybrids like Pow Wow Wildberry. They need more water than most of the other flowers I grow or the leaves and petals will develop black edges in the heat, but if watered well, they do pretty well.
Celosia spicata. Similar to coneflowers, they need more water, but if they get it they remain very happy even in full sun. I just lost one this week and I'm not sure why. Maybe it did get too dry. I have about 25-30 others that are fine though.
Salvia farinacea. This is a blue-flowered wildflower type salvia. When I was growing up in Texas, we called it clary sage. It can be either an annual or a perennial, and I think that for me it usually functions more like a reseeding annual, but I have one plant in one bed that has been there since about 2006. Smaller leaves and smaller flowers than one salvias, but I believe that is what enables it to survive so well in the heat.
Gazania daisies are very tough and tolerate a lot of heat and some drought if in very well drained soil. They aren't overly fond of my clay soil (even well-amended clay) so I don't grow them often.
I hope this gives you some ideas. People who live closer to your part of the state and who have conditions more like yours might have even more suggestions than the ones that do so well for me down here near the Red River.
Dawn
Q