"I know all you Federal boys (Chase and Mofed for instance) will say that Sharpsburg was a Union victory. But actually I'd say it was a "resounding" Confederate victory"

I would say it was definitely a tactical Confederate victory, but a strategic Federal victory.

Without a doubt, Lee and his lieutenants handled the ANV beautifully, maneuvering to meet every threat. Of course, McClellan's timidity in failing to attack for two days when the ANV was at its weakest and keeping 1/3 of the AOP on the sidelines on the 17th helped quite a bit. But taking advantage of opportunities presented by an opponent's mistakes is what makes a great general and Lee did it to perfection.

In spite of McClellan, it was a Federal strategic victory because it turned back Lee's invasion of Maryland, creating a trifecta in conjunction with the victories at Perryville and Corinth; the expected groundswell of support in Maryland for the Southern cause never materialized; the ANV suffered very high casualties that were not easily replaced and, most importantly, it gave Lincoln the opportunity to issue the Emancipation Proclamation which killed Southern hopes of European recognition/intervention and opened the door to the enlistment of USCT.

A "resounding" Confederate victory would have been routing the AOP from the field. As it was, Lee lost the initiative and had no alternative but to retreat into Virginia.