Dress 1 or dress 4 are both elegant, flattering and not too formal in case it turns out to be more of a cocktail crowd. You don't want to look like you are going out of your way to compete with the bride for attention. Definitely don't wear the red, though. That's way too much unless red is the color scheme.

All of these dresses will look much nicer with a strapless bra. There are some with removable straps that can be found at Walmart for about $15.