the only 100% effective is the new method of female sterilization that is called essure, but Mirena+Condoms is basically the same in practical terms. Essure is completely failure-free - no reported failures in the whole history of use.

essure also far surpasses the old female sterilization via tubal ligation because it is non-invasive, outpatient, etc. etc. - basically the best possible solution if you are really sure you do not want to get pregnant. But - also completely irreversible, while tubal ligation can sometimes be reversed.