Rabbi Yechezkel Landau, better known today after the name of his most famous book, Noda BeYehuda, was one of the greatest rabbis of his generation. He became the chief rabbi of Prague — a very prestigious position — while still a young man.
He was involved in the Emden-Eybeschutz controversy, opposed Sabbateanism, spoke out against the reliability and authenticity of the Zohar and writings of the Arizal, and attacked Chasidim who changed customs and prayers.
Many of his halakhic views are still relevant today. Most famously, he was the first to suggest that the kazayit should be much bigger than the size of an actual olive.