Fake any cup, but the bigger the better
Round shape, very firm, nipples placed high, or at least high enough (=usually don`t look nice if they are pointing to the ground).
As long as they look good, anything that makes them appear "faker" makes them also sexier in my book. I just happen to like silicone boobies and the more obvious their fakeness is, the more i like them.
But I like good naturals too.
Natural D-DDD cup, full, well shaped, not sagging.
Rare, but not impossible.