1. Roe v. Wade (not even close)
2. Clinton v. City of New York (I like the line item veto)
3. Florida v. HHS (Don't like Obamacare)
4. Griggs v. Duke Power Company (I think standardized tests should be allowed for employment consideration, as this could've prevented a lot of the credential inflation we've seen since, fueling the self-inflicted college loan bubble)
In light of what's happened to America since and its incredible secularization, Engel v. Vitale is a very tempting one as well. I'd also consider Bush v. Gore (way too much partisan divisions happened because of this, even though I'm quite conflicted on the case itself) and Citizens United (same thing).
I'm generally not a fan of standardized tests for employment, but I can imagine an employer trying to comply with both Griggs v Duke Power and Ricci v DeStefano at the same time must be a nightmare. Seems like if you want to hire people in any field you need both an expert in anti-discrimination law and a statistician to measure disparate impact and effectiveness of your employment practices.