TinyMCE Editor Set Default Tab

I came across this page — http://wp-snippets.com/set-default-editor/ * — which looks promising, though a little out of date. Instead of their code, I would suggest trying this:

add_filter( 'wp_default_editor', 'wpse101200_default_editor' );
function wpse101200_default_editor( $editor ) {
    return 'tinymce';
}

Reference

WPSeek’s wp_default_editor() page
wp_default_editor() in the WordPress core source

Unfortunately, the Codex doesn’t currently have a wp_default_editor() page.

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* Apparently so did you. I missed that the first time I read your question.