I've watched a few documentaries on this and given this a bit of thought and think that there are a few key points that lead to Germany being unable to win the war.
The Battle of Britain: The RAF was on the verge of collapsing when some German bombers accidently, or 'accidently' (I don't know), bombed London causing Churchill to order the bombing of Berlin. Hitler then retaliated and the RAF airfields were given a break whilst the blitz went on. I feel this is moderately important because Germany may well have attempted a sea-borne invasion (Operation Sea Lion) for which the BoB was a precursor, had they got control of the air. Whilst I feel they would have ultimately lost or failed at, due to the quality of their navy and our navy, it would have surely rattled the UK and cost us even more merchant shipping supplies in the Atlantic or channel. Would we have had the confidence to rock up in North Africa resulting in a push through Italy if we had been beaten? If we did not, "Hitler's soft underbelly" would have remained intact and the Italians would still an axis power and german resources could stay on the eastern front...
Invasion of the Soviet Union (Operation Barbarossa): This is the big one I think. Hitler was on track with Army Group Centre to reach Moscow before winter struck but he swung this army south to take some oil fields (to help Army Group South), delaying the blitzkrieg to Moscow. Had Hitler kept up his blitzkrieg momentum and reached Moscow before it got Very Cold (and taken the city) would the SU have sued for peace or otherwise "been out"? Because Hitler would then have his leibensrealm (don't fancy googling how to spell that...) so he might have seriously tried negotiating peace with the UK (with the UK very much on the backfoot) as he considered the British not far below Arians. I think by this point he had already offered for us to bow out fairly gracefully but keep out Empire. Crucially, I don't think any D-Day plans would have been successful without the SU steamrolling through Eastern Europe in 1944. I really think this one move cost him the war more than any other.
Declaring War on the US / alignment with Japan: Hitler did not need to declare war on the US. I can sort of see why he did; the US were supplying the UK and SU with all sorts of arms in their own ships and deliberately placing their neutral ships in the way (e.g. Iceland). I don't know if the US would have joined in the European war if Hitler hadn't made the decision for them; they were anti-war in general and the pacific theatre was particularly savage so there might not be much appetite to join in. Would D-Day have been successful without US service men but with UK, Canada, Australia (plus smaller numbers of other countries)? I'm not sure...
Then there is the nuclear prospect. I recall from one documentary that Hitler was not that interested in nuclear technology, but would he have been able to strap one to the top of a V2? No idea about the mass of the early nuclear bombs. The other angle is that had D-Day failed or not gone ahead at all, could we have "borrowed" a nuke from the US? I would have thought yes, and that may well have ended the war regardless of the year.
There are so many scenarios that could alter the 20th century beyond recognition...
I have always wondered why Hitler was not interested in the A bomb. I mean the Germans did think that the A bomb was some years away and they had more pressing needs, but it's not like Hitler to just miss an opportunity like that. Maybe he just didn't grasp how powerful the technology was. I guess it was kinda hard to believe that a five ton bomb could flatten a city...until you actually see one do it.