var app = angular.module('plunker', []);

app.controller('MainCtrl', function($scope) {
  $scope.item = 0;

    $scope.toggle = function(itemPos) {
       if ($scope.item === itemPos) {
           $scope.item = 0;
       }
       else {
           $scope.item = itemPos;
       }
    }
});
<!DOCTYPE html>
<html ng-app="plunker">

  <head>
    <meta charset="utf-8" />
    <title>AngularJS Plunker</title>
    <script>document.write('<base href="' + document.location + '" />');</script>
    <link rel="stylesheet" href="style.css" />
    <script data-require="angular.js@1.4.x" src="https://code.angularjs.org/1.4.3/angular.js" data-semver="1.4.3"></script>
    <script src="app.js"></script>
  </head>

  <body ng-controller="MainCtrl">
    <div>
      <div class="dropdon" ng-show="item === 1"></div>
      <div class="dropdon" ng-show="item === 2"></div>
    </div>

    <ul class="nav">                             
      <li>
        <div class="btn" ng-click="toggle(1)" ng-class="{ active:item1 }"><span>Nr1</span></div>
      </li>

      <li>
        <div class="btn" ng-click="toggle(2)" ng-class="{ active:item2}"><span>Nr2</span></div>
      </li>
    </ul>
    
    {{ item }}
  </body>

</html>
/* Put your css in here */