To what extent has Congress become the weakest of the three branches of US government?

Congress has become the weakest branch of government. Traditionally Congress has exercised the right to enforce decisions made by the Supreme Court, which would suggest that they are more powerful. However recent partisan gridlock has seen the Court adopt a quasi-legislative function which gives it legislative precedent over both chambers. Congress does retain some power over the president's domestic agenda, however its current composition has rendered it ineffective in checking presidential power. Moreover Congress has always remained weak at checking the president's power over foreign policy. These developments have indeed seen Congress become the weakest branch of government. 

The Supreme Court in 1803 granted themselves the power of judicial review. Though a long time ago, this is a development which has seen the Court be able to rule law as unconstitutional, such as in 2004 when the Court ruled in favour of Guantanamo prisoner Hamdi being allowed access to a citizen trial, something Congress had denied. This would suggest the court have become more powerful than Congress ever since the development of judicial review in 1803. However Congress reserves the constitutional right to enforce law and therefore they can ignore Court decisions, such as in 2004, 2006 & 2008 when Congress denied Guantanamo prisoners the right to a citizen jury despite three Court rulings all in the same vein. This power of enforcement has traditionally seen Congress retain political power in the face of judicial review and therefore they do not appear to have become the weakest branch of government.