Add category custom attribute with dropdown

Try this:

$this->startSetup();
$this->addAttribute('catalog_category', 'top_brand', array(
    'group'                => 'General',
    'type'              => 'int',//can be int, varchar, decimal, text, datetime
    'backend'           => '',
    'frontend_input'    => '',
    'frontend'          => '',
    'label'             => 'Top Hersteller',
    'input'             => 'select', //text, textarea, select, file, image, multilselect
    'default' => array(0),
    'class'             => '',
    'source'            => 'eav/entity_attribute_source_boolean',//this is necessary for select and multilelect, for the rest leave it blank
    'global'             => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,//scope can be SCOPE_STORE or SCOPE_GLOBAL or SCOPE_WEBSITE
    'visible'           => true,
    'frontend_class'     => '',
    'required'          => false,//or true
    'user_defined'      => true,
    'default'           => '',
    'position'            => 100,//any number will do
));
$this->endSetup();

I added eav/entity_attribute_source_boolean as source for your attribute.

Try to use below code to create top_brand attribute in category:

   $this->addAttribute( 'catalog_category', 'top_brand', array(
                'group' => 'General',
                'type' => 'tinyint',
                'backend' => '',
                'frontend' => '',
                'label' => 'Top Hersteller',
                'input' => 'boolean',
                'source' => 'eav/entity_attribute_source_boolean',
                'global' => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_GLOBAL,
                'visible' => true,
                'required' => false,
                'user_defined' => true,
                'default' => '', 
                'unique' => false, 
            ) );