Hmmm, my
favorite would be shapely A or B... I DO like them on the smaller side.
BUT, that's just my personal preference and I really try not to let it take control of my actual life. If I met a girl whose personality matched mine perfectly, and she had a pair of size-D breasts, I would love her with all my heart.
Oh, we're talking about PORN, are we... Well, in that case, let's just say a pair of shapely A-cup or B-cup... With certain exceptions, of course.