Let's do this:
I think early wartime Hitler from 1942-43 would likely be the best IMHO. His behavior will be more and more erratic towards the final years of his life but there has to be a solid middle ground where he has the experience that can offer him foresight into the future (despite his obvious bias) without the growing mental issues he was experiencing that was just growing worse and worse.
...but take the late part of "early", right as Stalingrad is starting to go bad. Hitler is already bitter and has the Holocaust going, but he's not truly pickled on drugs and delusion yet.
So let's say that it Hitler's consciousness from February 15th, 1943 that comes back in time. He's run the Third Reich state for almost ten years, a war with some stunning early successes for about three years, but he currently smarting and bitter over the recent defeat and surrender of "Field Marshal" Von Paulus to the Soviet Untermenschen at Stalingrad.
He goes back in time to the Germany of November 15th, 1917.
We can consider both versions:
I. He "parachutes" into the Kaiser, and is formally the sovereign "boss" of Hindy and Ludy, although they are not used to taking daily input from him. They find the Kaiser is suddenly heavily involved, supportive of all their militant instincts and hardline wishes, and anti-peacenik. He seems a much more serious man. They mostly like the changes, even if they don't always like the individual ideas of the Kaiser, and they notice a greater relentlessness and spine from the Kaiser against civilian politicians definitely, but also bureaucrats and military commanders who do not do what he wants.
II. He "parachutes" into Ludendorff and is managing the day-to-day war and administration, entrusted by the Kaiser and Hindenburg. He is used to strategic wartime management, along with making operational and even tactical interventions. He needs to adjust his thinking to avoid demanding weapons that are not available in 1917 or possible in with the technology and engineering of 1917. To a certain extent for more detailed operational, tactical, and logistical management to implement his strategic guidance, he can rely on Max Hoffmann and other key members of the German General Staff. He can also cooperate with right-wing politicians and manipulate Hindenburg and the Kaiser to a significant extent and Chancellor Hertling to minimize and sideline unwelcome Reichstag interference. He can keep his nerve until the end, so it can outlast that of the real-life Ludendorff.
I figure Hitler's priorities would be to:
1) Somehow get the spring 1918 offensive "right" and "win" there before the Allied tide becomes invincible. I do not know how he does that. Perhaps, in line with his racial rankings and estimations as much as with seeing operational opportunities differently, he might choose to throw the whole weight of German forces against the longer French sectors of the line to open up a broad front into the French interior, take Paris, or basically irrevocably crush the French Army, rather than focus on the British in Flanders. Any of these military plans could fail.
2) Set up an internal military, naval and state policing system to prevent major German mutiny or revolution like in November 1918, and to the extent possible, spread this to cover the other Central Powers as well. So setting up some sort of proto-SS and Gestapo organizations to make socialist and pacifist agitators disappear and shoot deserters, and crack down on the black market.
3) Get as good a start as possible on racial politics, propaganda, discrimination, looting, expropriation and murder of racial "undesirables". Get this started in November 1917. The Balfour Declaration can be used to begin portraying at least some segments of Jewish populace as agents who made a deal with the British in German, Austrian and Ottoman lands in particular, the prominence of Jews among Bolsheviks could support a theory of Judeo-Bolshevism or Communism as a mainly Jewish conspiracy, and cases of Jewish individuals involvement in black market scandals can be blown out of proportion to further stoke things. Since plenty of Jews are serving honorably in the armed forces, many distinctions will have to be publicly made pointing to exceptions and people exempt from persecution...for now...to reduce complaints or pushback. But people in "bad" categories can be robbed, imprisoned for hard labor or expelled to remote areas or Allied lines. If the Hitler as the Kaiser or Ludendorff has sufficient influence over Austria-Hungary he could launch preemptive repression and expropriation against Slavic minorities he knows will become disloyal to the Habsburgs. He could take similar action against the Poles and Lithuanians, or refrain for now, using them as tools against the Russians, as a fan of Pilsudski, but encouraging Polish and Lithuanian outrages against Jews in their midst.
4) Possibly over the course of the winter of 1917-18, or spring 1918, try to head off the Balkan offensive he knows will come in the fall by an overwhelming assault on the Allies boxed in to Salonica, possibly with heavy use of poison gas on the forces and civil population of the town?
5) Possibly acting differently toward the Bolsheviks. On the one hand, strategically there is just as much reason as in OTL to make the Brest-Litovsk peace, but Hitler as puppet master hates Communists. So he may just want to kill off Bolshevik leaders, and may tactically cooperate with some Russian Empire minority people or White Russians to do it, even if he has no intention of keeping any bargains with Slavic interlocutors.
The racial stuff can be a distracting waste, but to some extent, deciding that a segment of the urban and town non-farming population, either in German, or more likely in occupied areas, does not need to treated like human beings and fed as such and can be robbed or murdered by the state or their neighbors at will, could produce some minor "savings" in the face of blockade hardships.
If Hitler as puppetmaster, of either Ludendorff or the Kaiser, uses all his energy or influence to push something like this five-point program, how long can Germany last in this war? Can it last to 11 November, 1918? Would it collapse sooner? Could it last several months longer? Could it fight the Allies to a standstill?
How much murdering and displacement of targeted minorities - Jews, Gypsies, Slavs, could this regime accomplish in the year, more or less, it has to exist?
If the totals of regime racist crimes are high and a notable feature to the Allied powers disgust, does this have a notable impact on worldwide attitudes towards anti-semitism, genocide, and racism, from the 1920s and beyond, and the League of Nations's responsibility to deal with.
