Medium. But yeah...natural medium. Small ones aren't a turn on, big ones just ummm just wouldn't prefer them and jumbo ones are just a huge turn off. Natural medium breasts are just right.
Medium/big are wonderful and fine by me (my wife has what I would consider to be borderline medium/big, 34B...but when she puts on a little weight during the holidays, they're around 34C).
But, I do love the silicone-enhanced beauties too...