The Battle of Gettysburg is considered possibly the most crucial and is also the most deadly.

The First Battle of Bull Run constituted an early victory for Confederate forces.

Gettysburg was probably the most decisive because, it was a true victory for the Federals (Little Round Top, Culp's Hill and Picket's Charge...all Federal wins at Gettysburg). It was demoralizing to the South and the opposite for the people in the North because, it played bigger (than Vicksburg) in the big eastern papers as a battle which was won by the North. That alone raised Northern hopes for victory. It was a battle where Robert E. Lee was defeated and on Northern soil. He never tried to fight in the North after that. Gettysburg was Lee's major effort to win/end the war...though neither he nor the Federals planned it.