Name the major court cases that have challenged the Constitutionality of the Federal Reserve.
I think nobody filed a lawsuit, because it's obvious to most of us that Congress does in fact have authority to delegate responsibility.
If you don't believe that why don't you file a lawsuit then and see how far you get? Why doesn't Ron Paul?
Here is a synopses of the major court cases that have dealt with "delegation of powers". Generally the courts have ruled that they can delegate, they just can't delegate heir authority to the other branches. I don't know how comprehensive this is, it's the first one I googled.