Who division blocked Lee's retreat on its final day and received the first flag of truce from the Confederate force.

Custers Calvary under General Ords command.

The cavalry division of Brigadier General George Armstrong Custer seized a supply train and a number of guns at Appomattox Station, effectively blocking Lee's path because Appomattox Station was to the west of Appomattox Court House. Custer captured and burned the trains loaded with provisions for Lee's army. Interestingly Custer's brother, Captain Tom Custer, received the MoH in one of the battles of Lee's retreat as well. I've been to much if not all the pivotal Civil War battle grounds. From where Jeff Davis was captured to where Lee was captured,

The Battle of Midway early June 1942 changed the course of the War in the Pacific during WWII and put Japan in the defense instead of the offense.

The defeat of the Spanish Armada in 1588 by the British and Sir Francis Drake has to rate pretty high up there also.

Lord Nelson's Victory at Trafalgar.

The defeat of the British fleet under Thomas Gated by the French fleet under Compt DeGrasse in the battle of the Virginia Capes September 1781 virtually sealed the deal for Cornwallis' defeat at Yorktown. Thus eliminating his means of escape and bringing an end to the Anerican Revolution.

The Battle of Jutland in WWI.

Depends on your perspective, I can think of many more.