You could have a long discussion- or argument- about whether it should be the most famous, and also a long discussion about why it is the most famous. The fall of Vicksburg at the same time put the Union in control of the Mississippi and so much of the South, splitting the Confederacy in two and severely limiting its ability to import goods and move supplies. Even though he made his most famous speech at Gettysburg, Lincoln would likely have said Vicksburg was the more important of the two. There was also something of a Virginia bias in the Lost Cause historiography after the War, and a worship of Robert E. Lee, that made Gettysburg for them the biggest event. It also gave Lost Cause writers an argument to have among themselves as to whether Robert E. Lee ( who, after all was of Virginia) could have possibly have been the cause of the defeat or whether it was Longstreet ( who was merely from South Carolina) that deserved the blame.

However, it was very important. Gettysburg was a very big battle: over 100,000 Union and 75,000 Confederate soldiers, losses of around 23,000 on both sides. There were thus also a lot of famous events within it, like the defense of Little Round Top, and Pickett's Charge, that made for exciting reading ( Vicksburg, by contrast, was more of a slow slog). But it was also important because it was the decisive defeat of the larger Southern strategy to threaten the North ( especially Washington) and force it to negotiate for terms. Lee had made one earlier foray into the north in 1862, that ended at the Battle of Antietam. Gettysburg ended his second and last invasion. After Gettysburg the South would never again be able to launch an offensive into the North, would never really threaten it, and the remaining battles of the War were fought in the South, to defeat the South.