Most Helpful Guy
I don't know much about dresses, but I'll go with the third one. It has a nice feeling. Something casual, simple and comfortable. Should work on pretty much any girl, and it will still look good. It's maybe not the most extravagant option, but it's the safest. The one that, at the moment, speaks to me the most. I would see my future girlfriend wearing that, I might even buy it for her.
Most Helpful Girl
I think a couple of these pictured are actually swimsuit cover ups, as oppose to dresses.