TYPO3 Core

You can configure the key "priority" in the $EM_CONF of an extension. According to the current TYPO3 documentation, "top" and "bottom" and "" are allowed values for this field. The documentation says:

This tells the EM to try to put the extensions as the very first in the list.

(see http://docs.typo3.org/typo3cms/CoreApiReference/ExtensionArchitecture/DeclarationFile/Index.html)

I've tested this with a simple extension which is configured with "priority" => "top". In TYPO3 4.7, the extension manager installs the extension and inserts it at the beginning of $TYPO3_CONF_VARS['EXT']['extList']. In TYPO3 6.1, the extension management utility puts the newly installed extension at the end of ['EXT']['extListArray'] in the LocalConfiguration.php.

I think there is something wrong in \TYPO3\CMS\Core\Utility\ExtensionManagementUtility in the method loadExtension($extensionKey):

static public function loadExtension($extensionKey) {
    if (static::isLoaded($extensionKey)) {
        throw new \RuntimeException('Extension already loaded', 1342345486);
    }
    $extList = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance('TYPO3\\CMS\\Core\\Configuration\\ConfigurationManager')->getLocalConfigurationValueByPath('EXT/extListArray');
    $extList[] = $extensionKey;
    static::writeNewExtensionList($extList);
}

It seems to me that the "priority" setting of an extension is simply ignored at the moment.