I suggest following campaings:
1934 (After Hitler have choiced Head of Germany**),
1938(After Anchluss),
1940(After France Fallen),
1943(After Stalingrad Fallen),
1945 (After Germany Fallen).
Why 1934?
-Hitler have just become Head of State in Germany and you can start super-Military-Production if you play Germany
exam: you get time to do more Panzers, Maybe Transportships.
-If you play some Allies nation you can reckon better for war.
exam: With France you can Build Full Maginot Line in Belgium Border and more troops!
-In Asia China can recon better for war, if Japan attacks.
Why 1938
-If you wan't start fast before war, this is good for that. (France) Last year to improve Military, Fortications, (UK) Last year to improve HMS. (Finland) Last Year to improve troops in the *Winter War

*.
Why 1940
-(Germany, Hungary, Romania, Finland) If you wanna improve coming Operation Barbarossa plan this is last year for it!
-With Germany you can try *Battle of Britain* and try conquer England
with Operation Sea Lion.
-(Allies) Can fast conquer North-Africa before Axis get more Troops there, Before *Desert Fox* comes...
Why 1943?
-(Germany) Stalingrad have just Fallen, the believe of Germany Unstoppable army has gone, but Continuous withdrawing haven't start you can try Win Kursks Battles and make 1943 Summer Offensive!
-(Allies) You can Conquer Southern Italy with coming Operation Husky!
-(Comintern) It's time to payback- revenge of last two years!
Why 1945?
-(Allies) It's time to Invasion of Japan.
,,,After Germany fall you can start rebuild The Demoratic West Europe!
-(Comintern) After Germany fall it's time to attack Japan. What about support of Chinas Civil War?
-(Asia) Chinas Civil War
There's a few explanatory.
