What was most important battle of world war 1?

Well, its a matter of opinion, of course. I think a strong case could be made that the First Battle of the Marne was the most important WWI battle. This was the battle which stopped the initial German onslaught, saved Paris, and was followed by the "race to the sea" and the digging of the trenches. Had the French not been able to muster sufficient forces to fight this battle, or had they lost, the Germans could have defeated France in the first months of the war, which was their plan. The British Expeditionary Force was already having difficulties keeping a connection with the end of the French line, and had discussed withdrawing from the continent, a WWI Dunkirk. Amphibious operations would be necessary for a return to the European mainland, but the art of seaborne landings was undeveloped, as the British would demonstrate at Gallipoli in 1915. The British Army was relatively small, in any case. Throughout the war the British held only 50-70 miles of the 450 of opposing trenches. Without the much larger French Army to assist the outcome of continued warfare after a French defeat in 1914 would be unpromising.