These are all great choices. Personally, I don't like the light blue dress. But it's not a bad dress. You should take into account what looks extra special on you, or if you can't try it on, which colors really pop on you. I chose a color that went great with my skin tone, a cut that fit my body right, and overall something comfortable. Again, I don't know if you can try these on or not. But think about past experiences with dresses and go with your gut. All of these are very nice though, so no worries that any decision will be bad.
I like the cut of the green dress the best. The bodice of the darkest dress the best. And overall feel as if the first blue dress is where everything fits all together. My only take back about the green dress is that particular shade of green.
i really like the first and the second! if you're flat chested i suggest the second one because strapless dresses can be a challenge if you have no boobs (i made that mistake for junior prom lol). but if you have the boobs for it/aren't worried about it sliding down i really like that first one!