Where should non-member operator overloads be placed?

I want to overload operator<< for my class. Should I add this overloaded definition to the std namespace? (since the ostream operator<< is part of the std namespace) Or should I just leave it in the global namespace?

In short:

class MyClass {

};

namespace std {
    ostream& operator<< ( ostream& Ostr, const MyClass& MyType ) {}
}

OR

class MyClass {

};

std::ostream& operator<< ( std::ostream& Ostr, const MyClass& MyType ) {}

Which is more appropriate and why? Thanks in advance for your responses.