$cat = $_product->getCategoryIds();
$name= Mage::getModel('catalog/category')->load( end($cat) )->getName();

$categoryIds = $product->getCategoryIds();
$collection = Mage::getModel('catalog/category')->getCollection()
        ->addAttributeToSelect('name')
        ->addAttributeToFilter('entity_id', $categoryIds)
        ->addAttributeToFilter('is_active', 1);
$collection->getSelect()->order('level DESC')->limit(1);

$name = $collection->getFirstItem()->getName();