Actually Midway was the turning point, given how badly the Japanese Imperial Navy was whipped (they lost a lot of ships during that battle). The defeat effectively neutralized the Japanese threat in the Pacific Ocean; they would not be able to reinforce key areas (like Saipan, Guadalcanal, Solomon's, etc).
Saipan was a major loss for the Japanese as well, but it was due to the asskicking we levied on the Japs a few years prior.
It also marked a turning point in MacArthur's "island hopping" plan, paving the way for the invasion of Okinawa, Iwo Jima, etc etc.
Ordinary