To zero in on a single most important website feature is challenging as there are so many layers to consider. On one hand, I would say that the security features are most important because if the site were hacked and the participants’ private information is compromised, by word of mouth and online reviews, your business could tank. Beyond the install one must be mindful of secure passwords, keeping everything updated, avoiding plugins that are not necessary and not updated regularly, and installing an SSL to encrypt the connection between a web server (a website) and the client’s browser (Krol, n.d.)
Król, Karol. WordPress 5 Complete: Build beautiful and feature-rich websites from scratch, 7th Edition (p. 162). Packt Publishing. Kindle Edition.