Solution 1:

You can use the ng-selected directive on the option elements. It takes expression that if truthy will set the selected property.

In this case:

<option ng-selected="data.unit == item.id" 
        ng-repeat="item in units" 
        ng-value="item.id">{{item.label}}</option>

Demo

angular.module("app",[]).controller("myCtrl",function($scope) {
    $scope.units = [
        {'id': 10, 'label': 'test1'},
        {'id': 27, 'label': 'test2'},
        {'id': 39, 'label': 'test3'},
    ]

        $scope.data = {
        'id': 1,
        'unit': 27
        }

});
<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-app="app" ng-controller="myCtrl">
    <select class="form-control" ng-change="unitChanged()" ng-model="data.unit">
         <option ng-selected="data.unit == item.id" ng-repeat="item in units" ng-value="item.id">{{item.label}}</option>
    </select>
</div>

Solution 2:

try the following code :

In your controller :

 function myCtrl ($scope) {
      $scope.units = [
         {'id': 10, 'label': 'test1'},
         {'id': 27, 'label': 'test2'},
         {'id': 39, 'label': 'test3'},
      ];

   $scope.data= $scope.units[0]; // Set by default the value "test1"
 };

In your page :

 <select ng-model="data" ng-options="opt as opt.label for opt in units ">
                </select>

Working Fiddle

Solution 3:

You dont need to define option tags, you can do this using the ngOptions directive: https://docs.angularjs.org/api/ng/directive/ngOptions

<select class="form-control" ng-change="unitChanged()" ng-model="data.unit" ng-options="unit.id as unit.label for unit in units"></select>

Solution 4:

However, ngOptions provides some benefits such as reducing memory and increasing speed by not creating a new scope for each repeated instance. angular docs

Alternative solution is use ng-init directive. You can specify function that will be initialize your default data.

$scope.init = function(){
            angular.forEach($scope.units, function(item){
                if(item.id === $scope.data.unit){
                    $scope.data.unit = item;
                }
            });
        } 

See jsfiddle