Welcome, I note this is your first post.

If you are a newcomer, working on the so called "best keywords" - whatever they are, or may be - it won't help you one bit.

Assume the best, most profitable keyword was "Blue Widgets"?

Well you would find that very long established sites, in their hundreds, have "Blue Widgets" well and truly covered.

Successful sites focus on quality content about a genre the webmaster is an authority on. The webmaster doesn't pepper the site with "supposed popular keywords".

I suspect in the modern era Google doesn't even take keywords into account. Most people search by using phrases.

"How to paint blue widgets green?"
"Can I bake blue widgets in an oven?"
"How old are blue widgets before they can be picked?"
"How to propagate blue widgets?"

Google search will present the best pages they can. On those pages Google AdSense will present the searcher with the best, most relevant ads [subject to stupid interest based ads].