How to improve website ranking with on-page SEO?
What is the most important factor in on-page SEO?
website speed? keyword density? or other
Single most important factor? Hard to tell. This would be more of a guessing than valid facts.
Anyway, I think that markup and site navigation is fundamental.
Semantics are truly the base of your content. Markup is strong tool not just because it makes your content easier to read and understand. It also gives you great power of working with importance of provided information.
You decide what is heading, which text should be emphasized, bold, smaller than the rest... With proper markup and navigation, you can get the right content in front of searchers which brings you better CTR, lower bounce rate and longer time spent on your site. Which is what you really want.
Google will notice your visitors spend much time on your site, and they interact with it. That creates snowball effect and your ranks should improve.
Don't focus on keyword density too much. Keep it readable. Sometimes, less is more. And more is less...
One of the important factors to improve On Page SEO is Image Optimization.