Hans v. Louisiana
California Democratic Party v. Jones
Interesting, unusual choices.
Hans v. Louisiana because I think the history of the 11th amendment and the whole concept of state sovereign immunity is silly to begin with, and Hans set off a line of cases that expanded sovereign immunity in a way that makes nonsense of the text of the 11th amendment.
California Democratic Party v. Jones because... ok I'll admit, I couldn't think of another landmark case off the top of my head, so I went with a wonky topic that I recently came across in some research I'm doing. But I do think that in that case the Court overstepped its bounds in limiting states' ability to experiment with different electoral systems, and the ruling forces states to give greater recognition and special protection to political parties than I'm generally comfortable with.