The biggest, but it doesn't count as a subsidy is tying the dollar to oil. It keeps our prices relevant on the world stage. they have also fought several wars to guarantee that favorable trade position.
Another not-a-subsidy but still pulling favorites is when the prices get high and they cut the taxes. except for school supplies I can't think of anything that gets that deal.
Fracking and drilling on public land also receives favorable, not market, rates. The recent kerfuffle with Biden just capped that favoritism, it didn't remove it.
$4 billion in tax breaks a year:
otherwise, according to this it is 40 billion:
As President Obama and Congress look for budget cuts, some experts say federal energy subsidies are ripe for trimming. Among oil companies, nuclear power, and coal, who gets what from US taxpayers?
www.csmonitor.com
"Adjusted for inflation, they currently receive about $41 billion in annual subsidies annually. That amounts to more than half – 52 percent – of total benefits distributed to energy sectors by the federal government."
oh and that article brings up something I didn't even think of: ethanol.