We have discussed Gettysburg, but surely Antietam was just a important?
In its way, you're right. Both stopped Confederate invasions of the North. i think what makes Gettysburg stand out is that Antietam was essentially fought to a drew on the ground, but halted the Confederate advance, and Gettysburg was a convincing Union victory that not only stopped an advance, but, combined with Vicksburg, pretty much sealed the Confederacy's fate. All that remained was for stubbornness to kill another couple of hundred thousand men.
I agree. The Confederates never left the South again.
I was also considering how Antietam gave Lincoln the opportunity to issue The Emancipation Proclamation. One immediate effect was to block the South's attempts at gaining international recognition.