f4.5 best aperture for this shot, IMHO...the bulk of the flower is in focus, with the background still blown out. Did try this with a larger aperture, but it started looking more like a blob than a flower.
f14 - ya know, I think somewhere in the archives I have another shot of this kind of flower (echinecea?) I like this aperture IF there's nothing distracting behind it...but it doesn't work here with the background.