Best Keyword density

Good keyword density 1% to 2%.

 

For experimental purposes I just looked up the #1 spot for about 20 of my more recent target keywords ranging from 20-42 MoZ It was a wide range from about 0.8% and one oddity was 3.41%...

CONCLUSION:Google DGAF so long as you say it enough times that it deems your article relevant to your target key term, and not so much that you're a spammer.

 

It's okay to keep density 0.5-2.5% as a rule of thumb, nothing more.

Focus on the context and the idea behind the writing. I've seen highly ranked pages with lower than 0.5% density. Focus on the point and what you have to say.

Though, it may depend on the locale you are targeting also. While Google ranking systems is quite advanced in English language, I've seen different results on Indonesia locale, where keyword stuffing sometimes works

 

Keyword density is 2-3% of total content

 

I got a good kick out of reading all the responses but everyone was just guessing. Keyword density is real and does matter.
What you have to understand is that the way you're looking at it is all wrong. You can't use 1 keyword density percentage for every page, it's ALWAYS on a page-by-page basis. What's the correct density on one article/topic is not necessary the same on the next.

To KNOW what keyword density percentage Google is looking for, see what density percentage the 1st page ranking pages(for keywords) are using. These are the pages/density that Google is rewarding sites for and telling you they want/accept. If your density is way lower (under optimized), then you're not going to rank as well as you could. Have way higher (over optimized) density and you will not rank as well either. It all depends on the keywords/topic that's currently ranking 1st page.

Yoast plugin gives a generic percentage for all keywords or a "rule of thumb".