Michelle, I just read through all the comments, though I was going to vote for #4 from the start --- yes, cobalt is delightful on you, but that last purple gown is a whole class apart in terms of elegance and flattering proportions. Also, having 'seen' you in black on your birthday and admired you in blue here, I'm pretty sure purple will be smashing on you too!
From your last remark, you sounded unsure of its waist definition (this is #4, the purple); but I actually think it does define the waist in a far subtler and a far more effective way than the others. The long vertical lines of the skirt draping straight down and the narrow V and the diagonal pleating across the width of the bodice together converge on the jewelled trim at front waist, and the result is surprisingly hourglass.
I also just checked it out online from your link, and I love that it has a nice V in the back and is otherwise smooth too. Too many evening gowns make one look wider from the back because of a too-high neckline or a wide scoop or square. This one, I think, is a winner in this regard.
I certainly vote keeping this as your failsafe, in case you don't find something even more stellar.