A few brief thoughts in response to Milbarge's post below.
Gonzales v. O Centro Espirita is one of my candidates for a most-underrated case. It undoes half of the damage some people thought was done by City of Boerne v. Flores, when the Court struck down the Religious Freedom Restoration Act on enumerated-powers grounds. O Centro says that RFRA is alive and well, laws that substantially burden religious exercise must meet strict scrutiny, and the say-so of the litigating branch of the DOJ is not enough to count as a compelling state interest. (Neither is the say-so of a few folks in the state department about our treat obligations.)
O Centro is also big for what it might signal about the Supreme Court's attitude towards vice policy. To establish a compelling state interest sufficient to justify criminalizing a religious sacrament, the state has to show that the drug in question is harmful in the circumstances in question. It's not enough that Congress declared it to be harmful across the board. Of course, since a huge number of these drugs really aren't very harmful, that's going to make it very difficult for government litigators to prove that they are harmful in the particular cases at hand. Basically O Centro forces the government to prove on an individualized basis what isn't even true on a generalized basis, which may end up poking some holes in the current broad-blanket attitude towards the criminalization of the consensual ingestion of substances.
Of course, I'm enough of a realist not to imagine this attitude will hold for long; the deference shown toward Congressional factfinding last term in Raich is just one recent example. And I'm also not sure that this attitude is the proper legal posture for the Supreme Court to take. But it seems to me that simultaneously showing that the Religious Freedom Restoration Act has teeth, and that the government is not going to get a blank check where drugs are concerned, are two very big deals.
Central Virginia Community College v. Katz is my other candidate for an under-rated case, since it held-- quite implausibly given the caselaw-- that Congress can use an Article 1 enumerated power to abrogate state sovereign immunity. Even the lawyer who argued the case was positive that he was going to lose. Justice O'Connor's switch-in-time is a much bigger deal than the Court's decision in Georgia; it would be a bigger deal if she were still on the Court. Even Linda Greenhouse forgot about the case when she was reporting on sovereign immunity.
So many cases are over-rated that it's hard to know where to begin.