While Gettysburg is easily the most fascinating Battle of the war, I’ve always considered Chattanooga to be the turning point. That’s the Union victory that convinced Lincoln (and congress) to promote Grant to military commander in chief in charge of all union forces and for Lincoln to give him carte blanche in whatever he wanted to do. After that it was only a matter of time.
The fall of Atlanta was the beginning of the end. The war was unpopular in the South after that. It was mostly unpopular in the North for most of the entire war until then. Atlanta changed everything.
For the pure tactician I think Chancellorsville is the most studied battle, ever.