EUR 15.-Which are the 3 most frequently downloaded scientific articles in the internet?

Which are the 3 most frequently downloaded scientific articles in the internet?

I guess that could be infered referring to a citation index, however, I am paticularly interested in an article that is accessible via internet, ideally by indicating the number of donlwoads of that specific article.

The reward it given to the solver pointing me to the most frequently downloaded scientific article in the internet.