The ONLY correct way to write it, is the very first way I wrote it in the title.
WordPress
From version 3.0, WordPress included a filter in core that changed any instances of “WordPress” to “WordPress”, which would have included the title above.
This did break a lot of links and URLs that were in place in the content, excerpt and titles of posts. They modified the code somewhat, but the filter still exists, unless you install the plugin: https://wordpress.org/plugins/remove-wordpress-to-wordpress-filter/