Which would be the most important event or battle that led to Italy's unification?
Although this may seem odd, I would argue that the Crimean War was actually the most important event in Italian unification.
In this war, Piedmont (a state in what is now Northern Italy) went to war on the side of the French and British. This allowed Cavour, the prime minister of Piedmont, to push for Italian unification at the peace conference. He did not get it, but he did earn the respect and they sympathy of Napoleon III, ruler of France.
This was very important because it was the power of the French that did the most to bring Italian unification about. It was French help that allowed Piedmont to defeat Austria in 1859, which led to the acquisition of most of what is now Italy in the next year. Since there was no one turning point battle in the wars of unification, I would argue that the most important thing was getting France to take up the cause (even if it was sometimes a bit reluctant) of a unified Italy.