Each branch must consider the constitutionality of its actions but only the Court has the ultimate word on it. Nixon had no choice but to surrender the tapes when the Court ruled he must. Even the Executive must obey the Court or make a mockery of the concept of law. Only a Clinton would believe himself to be above the law. Are you claiming that the Executive has the right to defy the USSC's rulings if it believes them unconstitutional?
It is true presidents can veto bills but that can occur for many reasons. He can veto would-be laws which are perfectly constitutional as well as would-be laws for any reason whatsoever. It is true that if he vetos a would-be law and says it was because it was unconstitutional the Court has nothing to say about it. But it only rules on actual laws not semi-laws. Bills which are prevented from becoming law are not Laws.
However there is another aspect here which gives Congress ultimate control over the whole situation. Congress has the right to restrict the Court as to its jurisdiction under the Exceptions Clause. It never has used that power but that does not remove the clause from the Constitution. All the branches are equal but the Founders intended that Congress be slightly more equal than the rest.
"the ultimate word" You are out of time, I mean that view your are holding had its fifty-or-so-year run. A bad run, at that, some good needed gold and diamonds in the van but mostly a truck full of slask. (To use a swedish word.) It wasn't held prior and it shan't be held anymore. The Supreme Court has made too much a fool of itself to maintain that concept of final absolute authority. (A concept founded in sloth and hubris, both.) Read this:
... the famous Supreme Court decision Marbury v. Madison had in the past been interpreted as an absolutist claim that the Supreme Court is the final arbitrator of the Constitution. Modern scholarship recognizes that the famous decision merely argues that the Supreme Court’s duty to interpret the Constitution was the same as that of the Congress and the President. Far from demanding judicial supremacy, Chief Justice Marshall had asserted for the judiciary the same interpretative role of the popular will that the legislative and executive branches perform. (Source: Cornell Review, "Balance of Law Upset", http://www.cornellreview.org/viewart.cgi?num=265)
And not just "modern scholarship", but such was the case prior to the wool of sloth being pulled over the nation's legal eyes, back before the twenties.