Stay away from the breast. As a guy I have already seen way too many breasts that have been spoiled by a tattoo. God did a pretty good job on them just the way they are and a tattoo in that area just screws up what are normally very nice skin tones. Nothing ruins a good (or not so good for that matter) set of boobs quicker than a tattoo.
My vote is for the lower back if you have to have one, but I always really question why someone thinks they have to have one.
The only good answer I ever got was from someone with a bad case of turrets syndrome. He did a lot of tattoos and body piercing to distract people from his turrets syndrome. Unfortunateyl it did not work and in fact just highlighted things, but at least he had a semi-valid reason.
And I really don't care where you put a tattoo, they do not age well over a normal life span. So maybe if you have to get one, wait until you are 65-70, then it will still be looking relatively good when you go.