Marbury v Madison (1803) is still the most important Supreme Court decision, as it established the principle of judicial review. Judicial review means that courts can overturn or invalidate laws they view as violating the constitution. Obamacare has been challenged with several Supreme Court cases over the past decade. Supreme Court decisions overturned certain previsions of it (Burwell v Hobby Lobby, 2014) while others upheld some of its core provisions (King v Burwell, 2015). Marbury v Madison established the basic principle for Obamacare and other pieces of legislation to be challenged and partially or fully struck down by the courts. I hope this answers your question!
Burwell v. Hobby Lobby Stores. (n.d.). Oyez. Retrieved September 28, 2020, from https://www.oyez.org/cases/2013/13-354
King v. Burwell. (n.d.). Oyez. Retrieved September 28, 2020, from https://www.oyez.org/cases/2014/14-114
Marbury v. Madison. (n.d.). Oyez. Retrieved September 28, 2020, from https://www.oyez.org/cases/1789-1850/5us137