Overall, boxing is a good foundation for self defense but should be complimented by some type of grappling training (like Brazilian jiu jitsu, wrestling, or judo).
Muay Thai is likely the most effective striking based martial art while Krav Maga attempts to teach a defense system by combining various different martial art techniques. In most situations, Muay Thai would win over Krav Maga.
The best martial arts for self-defense is a combination of Brazilian Jiu jitsu, Muay Thai, and wrestling. An effective martial art should include live sparring and high level competition.