Best way of defining a compile-time constant

What's the best way of defining a simple constant value in C++11, such that there is no runtime penalty? For example: (not valid code)

// Not ideal, no type, easy to put in wrong spot and get weird errors
#define VALUE 123

// Ok, but integers only, and is it int, long, uint64_t or what?
enum {
     Value = 123
};

// Could be perfect, but does the variable take up memory at runtime?
constexpr unsigned int Value = 123;

class MyClass {
    // What about a constant that is only used within a class, so
    // as not to pollute the parent namespace?  Does this take up
    // memory every time the class is instantiated?  Does 'static'
    // change anything?
    constexpr unsigned int Value = 123;

    // What about a non-integer constant?
    constexpr const char* purpose = "example";
    std::string x;
    std::string metadata() { return this->x + (";purpose=" purpose); }
    // Here, the compiled code should only have one string
    // ";purpose=example" in it, and "example" on its own should not
    // be needed.
};