All of it is important.
Do whatever you can to give your site a boost above your competition. If your competition doesn't have keywords in their domain name... get one with keywords.
Use your meta tag keywords in your description, page text content and in your alt-img tags.
Every page on your site should have different content and meta tags, but relating to the same concept.
Something else I've noticed in the somewhat recent past, is the age of the domain name.
My older domains tend to do better in ranking than the ones I've recently purchased.
Anyhow, there isn't 1 thing to do that will give you the best ranking, which makes me wonder if you are really serious.
Is the decision to get a domain with or without a keyword in it even something to think about?? I mean you want to know "how much", but, even a slight increase in page rank is better than none. It seems to be a no-brainer in my book.
