I would say do not ignore any kind of errors, nor the elements that are connected to your SEO. They might not seem very important at that moment, but they can have a negative influence on your SEO efforts.
And if you ask me about three critical elements, then they would be:
- Site load speed - sites which take more time to load are not ranked good. So, Site load time should be less.
- I would say during the audit you should check if all search engines are able to crawl all the right pages of your website?
- The most important thing is to remove the duplicate content, if any.