<select ng-options="item.subItem as item.label for item in items track by item.id" ng-model="selected"></select>
Given this array of items on the $scope:
$scope.items = [{
id: 1,
label: 'aLabel',
subItem: { name: 'aSubItem' }
}, {
id: 2,
label: 'bLabel',
subItem: { name: 'bSubItem' }
}];
<select ng-options="item as item.label for item in items track by item.id" ng-model="selected"></select>