How do you know what's the best birth control?

IUD/Implant. Regardless of your age, blood pressure, family history, or reactions to other birth control, the most effective and safest methods of birth control are long-acting progestin-only methods, such as the implant or the hormonal IUD, which decrease rates of endometrial cancer and decrease bleeding. They're safe, reversible, and don't affect weight gain/mood. They're almost as good as getting sterilized.
The one you will use. Each hormonal birth control method is fairly similar in effectiveness. The best one for you is the one that you feel most comfortable with, tolerate the best, and will use consistently. If you freq. Forget the pill or are delayed in taking it, consider the NuvaRing ( change one x per month) or an iud or Implanon - lasts for several years.