What is the percentage of US federal income tax paid by non-US citizens?

That is, individuals who are not citizens of the United States but are
permanent residents (green card holders), students, or otherwise
working in the US on a visa and paying taxes to the federal
government?

Probably a difficult question to answer, considering many citizens are
married to green card holders and file joint returns.

Green card holders are US tax residents and are treated exactly as US citizens.

Students are non-residents for tax purposes and file 1040NR. Their tax
rates are basically the same as US residents except that they do not
enjoy some of the breaks that US residents get (e.g. they cannot file
jointly with spouse, they cannot receive a standard deduction etc).