General John Buford
Buford commanded the Union Cavalry brigades that the Confederates under Henry Heth, a division in A.P. Hill's Corps first struck as they marched toward Gettysburg on the morning of July 1, 1863.
It was Buford, as much as any other Federal officer who thought Gettysburg would be a good place to stand and fight.  His men held up the Rebels long enough for both Armies to converge on the town.