1st (best): Real (except in rare circumstances, e.g. certain congenital defects, post mastectomy etc.), even if small etc.
2nd: Real with some surgery (e.g. breast lift) but no artificial implants
3rd: Fake, generally the newer more natural shaped ones that aren't too firm (unlike no. 4)
4th (worst): Fake, old-school "stuck-on round boulders"