What is the default value for enum variable?

It is whatever member of the enumeration represents the value 0. Specifically, from the documentation:

The default value of an enum E is the value produced by the expression (E)0.

As an example, take the following enum:

enum E
{
    Foo, Bar, Baz, Quux
}

Without overriding the default values, printing default(E) returns Foo since it's the first-occurring element.

However, it is not always the case that 0 of an enum is represented by the first member. For example, if you do this:

enum F
{
    // Give each element a custom value
    Foo = 1, Bar = 2, Baz = 3, Quux = 0
}

Printing default(F) will give you Quux, not Foo.

If none of the elements in an enum G correspond to 0:

enum G
{
    Foo = 1, Bar = 2, Baz = 3, Quux = 4
}

default(G) returns literally 0, although its type remains as G (as quoted by the docs above, a cast to the given enum type).

I think it's quite dangerous to rely on the order of the values in a enum and to assume that the first is always the default. This would be good practice if you are concerned about protecting the default value.

enum E
{
    Foo = 0, Bar, Baz, Quux
}

Otherwise, all it takes is a careless refactor of the order and you've got a completely different default.