For Me it would be Stonewall Jackson. When He died,Lee and the Confederate army of Virginia wasn't nearly as strong
Militarily, the Army of Northern Virginia was at its peak when Stonewall died. And while Bevin Alexander has done a great amount or research on Stonewall's relation to any possible Confederate victory, his research largely hinges on the First Battle of Bull Run and the fact that the Confederacy didn't take a major offensive immediately after the Union rout.
While in pure theory, such an attack probably could have won the war, it is not a clear cut case. The Confederate army was just as green at 1st Bull Run as the Union, and it was EXTREMELY fortunate that Stonewall's men saved their position in that battle, because it very nearly became a rout in the other direction. As such, while Washington DC was completely undefended, there is no gaurantee that the Confederates could have maintained the discipline to take Washington rapidly and capture Lincoln and prominant members of the Union government as well...
Would the South have taken Washington? Yes, but whether or not Lincoln and prominant Union politicians would be captured is in question, and if Lincoln escapes, the war would continue into 1862, which would force the "battlefield calculus" to be done again...
And in history, since the Confederacy failed to attack Washington DC, Bull Run proved to by a hollow victory. Once McClellan trained the Army of the Potomac, the South would never again get the chance to have the enemy in complete and utter confusion fleeing before its forces.
