You can read more about the different defines in Zen Cart's configure.php. DIR_WS_CATALOG is the "folder" or "directory" where you installed Zen Cart relative to the Web Server's document root (or in other words what shows up following the domain name in your web browser's address bar). Note DIR_WS_CATALOG should start and end with a slash (/).

So if you have define('DIR_WS_CATALOG', '/'), then use either use: "RewriteBase /" or "#RewriteBase /".
If you have define('DIR_WS_CATALOG', '/shop/'), then use: "RewriteBase /shop/".