Which version of WordPress is best?
It is always recommended to use the latest WordPress version. So, as of today WordPress, 5.9 is the best version as it has improved performance, and is updated with all the fixes missing in the previous versions.
What is the risk of using an outdated WordPress version?
Outdated WordPress plugins can do damage to your website. Ignoring your WordPress update notifications may be one of the easiest things to do, but it's also probably the most damaging. Here is why: inconsistent updates can lead to serious security issues, compatibility problems, and land you in technical debt.
versions before gutenburg
version from 5.9 Full Site Editing
well, an old or new version must have a security plugin like wordfence, as the techology changes or you follow or you freeze the site with a non update plugin, always must have good backups becuase the bad people are always better and more clever. If you have a car of 2000, you must have an ecosystem to support it, similary is with WordPress.