To think that Germany could have won the war is a stretch, but won the war in Europe and held onto their gains is not too far fetched. If assassinated I believe the war would have been brought to a close much sooner, but if he wasn't and let the generals run the war the way they wanted, then a German mainland Europe would not have been out of the question.
Germany needed resources, they had as much as they could get with their dominance of mainland Europe, they needed oil though, they could have had this with taking the middle east. Their mistake here, Japan's bombing of Pearl Harbour and Germany's declaration of war on the U.S. As stated, in their Axis alliance, Germany should have pushed for Japan to invade Russia, where the resources they needed lay, coinciding with a German attack on the Soviets. Mistakes; Germany declaring war on the Americans, thus drawing them in and Germany going it alone in Russia. Further problems existed within the Russian campaign. Hitler taking charge is the biggest problem. Going it alone may have worked, had the generals been left to decide, they where not far from Moscow when things went wrong. They needed the Caucusus oil fields now because of the failure in the middle east, thereby spliting their forces. Hitlers obsession with Stalingrad, where the 6th army should have been allowed to pull out of, saving a solid fighting force, thusly setting up the next failures to come. Kursk being a big part of that. If the 6th army had been able to pull out of Stalingrad, they may have been able to consolidate and hold for the winter. Too many resources were spent on Stalingrad, setting up the German army in Russia for certain failure.
Earlier in the war, the Germans should have annihilated the Brits at Dunkirk, not in preparation for an invasion of Britain, but to buy time in their conquest of the rest of mainland Europe and then the Middle East. The U-boats and the big German battleships kept the Brits at bay. When the Brits did go after the big ships, quite an obsesssion, but for a reason, they lost alot of men and a few ships. The U-boats could have and would have kept the Brits from being able to wage war. They needed a stronger Italian ally to help in their conquest of the Middle East and part of Africa. Once this area is consolidated, then with Japan's help they could have turned east.
Some of Hitlers bigest mistakes were his meddling in tactics and plans. Wasting time on Britain, loosing too much time there and too many pilots. Not having good allies and a proper plan of action to succeed. Japan bringing the Americans into the war was the most crucial mistake made.