What is the best contraceptive for a teenager?

Depends on her. There is no one contraceptive that works best at any age. All birth control options except sterilization are reasonable for teens: condoms, diaphragm, pills, depo, implanon, iuds. It depends what she is willing to do. Some teens feel it is very hard to use the pill since it has to be taken every day. Depo (3 months), Implanon (3 years) and iuds (5 and 10 years) require less compliance.
One They Will Use. Contraceptive choices vary from person to person. It is best for the teen to discuss the possible choises for them with their doctor and the two of them determine what would be the best method for that individual, their lifestyle, beliefs, and other health issues.