Currently studying this and Supreme Court cases fascinate me. I ask my teachers but they always say there isn't just "one" important one. Maybe Reddit might find one that is most important?
It depends on your life.
If you've had an abortion, Roe v. Wade might've been most important to how your life turned out.
If you're African-American, Brown v. Board of Education probably had the biggest effect on your education.
Or maybe it's Marbury v. Madison, because many attribute that to creating the judicial review process at the Supreme Court level (though I've heard recently that it's debatable to claim that).
Maybe it's National Federation of Independent Business v. Sebelius, if you're uninsured and need healthcare that ignores preconditions when you apply.
If you've been arrested, and they didn't read your Miranda rights, maybe it was Miranda v. Arizona that got you out of trouble and let you keep your job.
If you're a conspiracy theorist, maybe you hold that US v. Nixon "proved" your point that all politicians are corrupt.
If you had to go to court for a major offense that you were framed for or wrongfully accused of, maybe Gideon v. Wainwright is the reason you got representation that could help you win the case.
Do you see why your teachers say that? Each person has different values. If you're looking for the biggest general impact, you have to determine which of the qualities of the Supreme Court cases listed above (and all of them in fact) have not only the widest spread, but also weigh that against how impactful it was across that spread. Simply put, it's impossible to measure. Reddit might like to debate stuff like that, but this is far too speculative for /r/AskHistorians I'd say.
I don't know why your teachers don't pick the most important one thats a cop out.
Marbury v. Madison sets up Judicial Review. Hence every decision the court made that involves an act of congress owes itself to Marbury v. Madison. It's most likely the first case any pre-law student will ever read and analyze for this very reason.
If you want to go hard mode and ignore that one then a case can be made that the infamous Dred Scott v. Sanford goes right up there. First its been cited by disenting judges countless times as a warning to not screw it up like they did with Dred, But also an argument can be made that Dred Scott was the first case to use the concept of Substantive Due Process or SDP. SDP was one of the argument in Griswold v. Connecticut which out of nowhere the Supreme court created the right to privacy. Something that most people think is in the constitution but its not. According to the SC majority decision and quoted from Wikipedia - Although the Bill of Rights does not explicitly mention "privacy", Justice William O. Douglas wrote for the majority that the right was to be found in the "penumbras" and "emanations" of other constitutional protections, such as the self-incrimination clause of the Fifth Amendment. After the creation (or acceptance depending on your POV) of Privacy as a right you have a straight line to Roe v. Wade aka abortion whose basis is the right to privacy from governmental interference with ones own body.