I see you already have many answers, how about image optimization, code optimization as well?

Complete list can be found on wecodeart.com/onpage-seo-basics/

Yes...site speed and responsivness is also a factor for search engines.

On page SEO aspects are:
Keyword Research
Title tag
Meta Description
Sitemap
URL Structure
Image Optimization
Update Content regularly
Internal linking
keyword Density
Headers Tag

Of these on page aspects wich are the most important?

Content of Page
Good content supplies a demand
Good content is linkable
Title Tag
URL

Onsite optimization is anything that you do within your website to optimize it for the search engines. There are key factors to consider here.

They are listed as follows:

The domain name.
The meta description.
Web contents.
Internal links.
Tags and metatags.
Anchor texts.
Rick media files.
Bounce rate.
Loading speed.

On page SEO aspects are:
Keyword Research
Title tags
Meta Description
Robots.txt
Sitemaps
URL Structure
Quality Content
Image Optimization
Internal Linking
Headers tag
Update Content Regularly

there are various on page aspects:
1. Title
2. Des
3. h1, h2, h3, h4, h5, h6
4. content
5. alt
6. sitemap
7. robots.txt