Actually, thinking in terms of largest total return for a project (ROI is infinite if your I is 0).. I can't think of too many projects that have had larger returns.

Probably the iPhone? Maybe some Cars (camry? Model T?) Maybe some really big mining/oil operations? What else is in the same league?

To be sure, there has been marginal investment over the years in maintaining absolute backward compatibility through the zSeries/zOS.