Not able to post comment anonymously: getting 403 error
Thanks  tyler.frankenstein. Thanks for suggesting angular-drupal. 
I integrated it with my agularjs ui and its working superb. 

I gone through angular-drupal code and found that it need X-CSRF-Token in header for write operation. Accordingly i modified my CORS setting 

*|*|POST,GET,PUT,DELETE|Origin, X-Requested-With, X-CSRF-Token, Content-Type, Accept|false


And used agular-drupal method to save comment. Here is my code. 

//register drupal service as provider/injector
.factory('$drupal',['drupal', function(drupal) {
  return drupal;
}])
.
.
.
//inside controller

.controller('HomeCtrl', function($scope, $stateParams,$http,$state,$drupal) {


  $drupal.node_load(27).then(function(node) {
    console.log(node.title);
    alert(node.title);
  });

  var comment = {
  nid: 27,
  subject: 'Hello world',
  comment_body: { und: [ { value: 'How are you?' } ] }
};
$drupal.comment_save(comment).then(function(data) {
    alert('Created comment: ' + data.cid);
});


Other way to achieve this without angular-drupal is to generate x-csrf-token using get request "/?q=services/session/token" and then set it in header for your post request. 

Thank you.