I prefer breasts on the small side ever since I learned in some sex book that small ones have just as many nerve endings as large ones do, making them more sensitive to the touch, the lick, etc. I like nipples on the small side, too.
As far as visual appeal, a woman whose breasts are in proportion is the best for me. Meaning, a petite woman with small to medium would look very small on a tall woman, and vice versa.