Answer:
Lexington
Why were the Revolutionary War battles in Lexington and Concord so important?
The actual combat started in Lexington.
Gage had been intimidating the peasants for months by leading marches of Regulars around the countryside. Gage's office leaked like a sieve, and the colonists had devised a complex system of alerting the militia, so everywhere the Regulars went, militia companies were ready to make sure they didn't create any trouble. To restrict where the Regulars could go, the militia like to climb boards off of bridges.
Gage dispatched an unusually disorganized team to Concord on April 19, 1775, to capture (i.e., steal) militia supplies. Maj. Pitcairn was dispatched ahead by Col. Smith to secure any bridges. There was a bifurcation in the road at Lexington but no bridge. To let the Regulars know they were there, Cap. Parker had the militia line up in parade ground formation adjacent to the road on the right fork. Pitcairn, however, gave orders for his forces to assault the militia rather than merely march to Concord (on the left fork). After that, he took the left branch and circled a building to get away from his charging Regulars. Parker ordered the militia to disperse despite being outnumbered 3 to 1. The upshot of the fighting was one Regular grazed and 20 militia losses, including Parker, who was killed.
In order to start the battle, Lexington was commanded to charge by a British officer who then promptly rode off in the opposite direction.
Later, Concord was significant because other militia forces fought the plundering Regulars in retaliation. This persisted all the way back to Boston's vicinity. The Regulars would not have returned if a relief column had not arrived (they were almost out of ammunition when they met the relief column in Lexington). It was obvious even then that the Regulars had been soundly crushed.
Concord therefore demonstrated that the American militia was capable of defeating the renowned British army, as did the five engagements along the road.
Three amusing facts First, in contrast to popular belief, the militia engaged in battle as coherent, well-trained formations rather than as lone combatants. Second, the British did not just march in a straight line; a large portion of their army consisted of light infantry, which could and did cover the flanks and engage in irregular warfare. Third, many of the militiamen in this engagement were French and Indian War and/or Pontiac's Rebellion combat veterans, whereas the Regulars in this conflict were inexperienced.