I believe in the following (though not ranked by importance)
1) Kursk-Although Stalingrad and Moscow were decisive, Kursk determined the entire outcome of the war. With the lost of Kursk, German hopes for a victory, or even a stalemate were lost completely
2) Leyte Gulf-Although Pearl Harbor and Midway were decisive, Leyte Gulf showed which fleet were superior and the Japanese losses were considerable. It was also like Kursk, which ultimately determined the fate of Japan
3) Of course, Stalingrad, Moscow, Midway, and Pearl Harbor were also decisive battles, with D-day and El Alamein being major, yet I believe those two battles sealed the war's fate to a point which the Germans and the Japanese realized they have lost
On the question of whether Germany had not attacked the Soviet Union: It is an impossibility. Even if Hitler had honored the pact (which was extremely unlikely due to ideological differences and hate for each other), Stalin wouldn't had. Stalin even wrote that if Germany hadn't invaded in 1941, the Soviet Union would have had in 1942, when all preparations for war would have been complete. Nazi and Communism differences were very pronounced, as Nazism were right-wing and Communism were left-wing. Not only that, but anti-Communism feelings were high even before the Eastern Europe campaign in Germany, and the signing of the Anti-Comintern pact by Germany showed how much hatred there were between the two countries