I agree with all the others that each of us will have our own figure flattery priorities and also, what counts as a summer dress in one climate might be too warm or covered up for summer in another climate.
My favourite summer dresses in hot and steamy Ontario were always sleeveless, always cotton or cotton-linen blends, and always loose fitting.
In cooler, breezier, less humid west coast summers, I still reach for those dresses (sometimes with the aid of a topper) but I can also wear dresses with sleeves if the fabrication and cut are cool and breezy enough.
My figure flattery priorities are -- well, it can't be too loose/ oversized, because it will swamp me, and it can't be too tight or skimpy, because I'll feel awful in it and look ridiculous. Also, most maxis are just too much for me. And minis don't suit me -- my thighs are not my best feature, ahem.
Bias cut dresses and very gathered skirts are usually not my friends. Knife pleats are tricky -- it just depends how they lie. But many give me too much tummy bulk.
I bought 2 summer dresses from the Rack last year. The grey and white stripe is full skirted and works best for "date night" type situations because it has a certain "crispness" to the feel (even though it's actually quite casual). Talk about a well-made garment, though. Lovely finishing on that baby.
The Free People is a fun item I'd never have worn in Ontario summers. Works here, though.
My other casual summer dresses are the blue and white Comrags, which is a kind of non-wrinkling seersucker cotton -- absolutely brilliant. It is dropwaisted and very breezy. I love this dress.
The J. Crew 2 piece gets worn in so many combinations. I actually ripped the skirt last year but sewed it up so it is not noticeable. There is elastic on the back of the waist so it never feels tight there. Top can be worn tucked or not. Very lightweight and does not show the wrinkles because of the pattern. It is cotton-lined cotton, one of those uncharacteristically super high quality J. Crew items that used to show up every now and again.
I would really like one or two "day dresses" -- super casual dresses that I can throw on and wear even in the garden, if need be. I'm on the lookout now. A dress has such a great feel to it in summer. Nothing better.