As Frank Zappa once said...
Mammalian protuberances!!!
Personally my two favorites are
rack and
juggs. Which one I use depends on the size. Rack works no matter what the size. Even if the girl is practically flat-chested, rack still works.
Juggs, on the other hand need to be somewhat sizeable. They don't have to be huge but they need to be fairly hefty.
For example, Jessica Biel has a nice rack while Jennifer Love Hewitt has nice juggs.