For me, neither the cup size nor the number. The breasts must fit the body, if that makes any sense. One could photoshop breasts from one woman to another and what looks good on one would appear out of place on another. Also, a woman's bosom that is supported and dressed can look very different when it is naked.
Despite her "issues" I've thought Lindsay Lohan's rack was awesome, same with Scarlett Johansson. And, as writer Dan Jenkins once wrote, a nice pair of nipples can set off a nice rack like a money clip sets off your whip out (cash, not your dick).