What would increase IRR the most: a $100M increase in revenue, a $100M decrease in costs, or a $100M decrease in net working capital?

A $100M decrease in net working capital would increase IRR the most.

A $100M increase in revenue would typically come with an increase in COGS as well as an increase in taxes. If the increase in revenue came from an increase in prices, then COGS would not increase, but taxes would still increase.

A $100M decrease in costs would increase pre-tax income and therefore increase taxes.

A $100M decrease in net working capital is not part of net income and therefore does not increase pre-tax income or increase taxes. It would increase cash flows by a full $100M, and therefore increase IRR the most.