Eh? Pretty sure the Union won at Shiloh. Do you mean a more complete victory, instead of just a defensive victory that drives off the Rebels? Overall best Pod...Hm. I would probably say a successful Peninsula Campaign or slightly better luck/more aggressive Little Mac at Antietam. Either of those things would have tipped the balance. If the Peninsula Campaign takes Richmond, that pretty much does away with the South's most important industrial center. It will become increasingly more difficult for them to fight as long as they don't have it. A better Antietam might just destroy the Army of Northern Virginia. Either would be decisive, the former in the long term, the latter in the short term.