Plugin set default setting value when it activated

Your code will certainly use get_option() to retrieve the values for your options. get_option() accepts a second argument that allows you to specify a default. Use that instead of inserted values into the database unnecessarily.

get_option( $option, $default );

If you are concerned about third party code, there is there is the option_{$option} filter that you should be able to use to preserve your default even then:

116         /**
117          * Filter the value of an existing option.
118          *
119          * The dynamic portion of the hook name, `$option`, refers to the option name.
120          *
121          * @since 1.5.0 As 'option_' . $setting
122          * @since 3.0.0
123          *
124          * @param mixed $value Value of the option. If stored serialized, it will be
125          *                     unserialized prior to being returned.
126          */
127         return apply_filters( 'option_' . $option, maybe_unserialize( $value ) );