WordPress

Enable user-plugin installation by allowing write access to wp-content/{plugins,themes} and configure:

# in wp-config.php
define('FS_METHOD', 'direct');

Other nifty things:

CLI, wp-cli.org
Security scanner, wpscan.org

Use SSL

Use Letsencrypt to enforce HTTPS everywhere and dehydrated to maintain the SSL certificate, and use the latest Mozilla SSL Guideline from their configurator to set up strong SSL.

Content security

Use a content security policy. There is a plugin for managing one from within WordPress, but it is straightforward to cobble one and add it to the headers. Here's an example content security policy that allows JavaScript, images and CSS from only certain origins, and denies everything else:

Header always set Content-Security-Policy "base-uri 'self';
     default-src 'none'; media-src 'self';
     connect-src 'self' https://*.wp.com wss://public-api.wordpress.com https://api.stripe.com
         https://checkout.stripe.com;
     font-src 'self' data: https://*.wp.com https://wordpress.com https://fonts.googleapis.com
         https://fonts.gstatic.com;
     form-action 'self';
     frame-ancestors 'self';
     frame-src 'self' https://jetpack.wordpress.com https://*.wp.com https://www.google.com
         https://*.stripe.com https://player.vimeo.com https://www.youtube.com
         https://www.surveymonkey.com;
     img-src 'self' data: https://*.tile.openstreetmap.org https://*.gravatar.com https://*.wp.com
         https://*.wordpress.com https://s.w.org https://maps.googleapis.com https://maps.gstatic.com
         https://q.stripe.com https://secure.surveymonkey.com;
     script-src 'self' 'unsafe-inline' 'unsafe-eval' https://widget.surveymonkey.com https://*.wp.com
         https://public-api.wordpress.com https://*.stripe.com https://maps.googleapis.com
         https://ajax.googleapis.com;
     style-src 'self' 'unsafe-inline' https://*.wp.com https://ajax.googleapis.com
         https://fonts.googleapis.com;"

Other headers can be used to further enforce some secure behaviour:

Header always set X-Content-Type-Options "nosniff"
Header always set X-XSS-Protection "1; mode=block"
Header always set Referrer-Policy "strict-origin-when-cross-origin"
Header always set Access-Control-Allow-Origin "*"

gzip + WP SuperCache = win

Lock things down

As suggested by WPScan, prevent DDoS attacks on web-accessible cron and XMLRPC endpoints, but allow CLI invocation, and prevent execution of uploaded PHP content:

<Files "xmlrpc.php">
    Require ip 127
</Files>
<Files "wp-cron.php">
    Require ip 127
</Files>
RedirectMatch 404 "/readme\.html"

<Location "/wp-content/uploads">
    SetHandler none
    RedirectMatch 404 ".+\.php"
 </Location>