Depo Povera seems the best http://www.bbc.co.uk/health/physical_health/sexual_health/contr_injections.shtml

I hear that Thai women often attribute a change in facial pigmentation, at times large dark areas, to oral contraceptives. Is that so? Any injection related observations?

Melasma (the condition you refer to) is often related to hormonal changes from either pregnancy or combined oral contraceptives. Using sunscreen can help reduce the occurrence of it as may using a lower dose pill OR a progestin only (as opposed to combined estrogen + progestin) pill . It does not occur in all women, only a minority of them, and goes away when the method is stopped. So I would not let fear of it prevent trying the pill if that method otherwise appeals.

Injectable contraceptives contain only a progestin and no estrogen and very rarely cause this problem, so many -- but not all - women who have melasma on oral pills are able to tolerate the injection. Ditto implants. Implants are the exact same drug as the injection, but long acting and releasing a steady low dose into the blood stream whereas the injection results in a "surge" level that gradually decreases. However since the implants require a minor surgical procedure to remove it is a good idea to first try either the injection or a progestin only brand of pill for a while to make sure the woman tolerates it well.

IUD is another alternative and has the advantage of being non-hormonal.

All of these methods have a number of contraindications and pros and cons apart from the melasma issue so should read up on it carefully before making a choice.