Which one do you think was the most impressive achievement by military forces in the 20th Century?
Impressive military achievements are usually due to an unexpected superiority in one or more of the following areas: planning, training/experience, tenacity/bravery, technology/equipment, intelligence, leadership, surprise, tactics, and strategy. However, this is usually accompanied by serious deficiencies on the other side as well. Luck also plays a major factor but is usually forgotten in place of one or more of the other positive factors.
I think you have an impressive list of candidates, but it is difficult to consider entire stretches of WW2 since these were effectively separate campaigns sometimes under the influence of different people and forces. So I suggest this list in order of achievement, for the following reasons:
German attack against the Western allies May 1940
Planning, Training/experience, tactics, strategy, leadership, so this has many of the elements. Scarred by the failure to cut of the British though.
The Israeli victory in the Six Days War, 1967
Training, technology, surprise, intelligence.
Communist forces against US and their allies in the second Indo china war (Vietnam)
Tenacity
The Finnish stand against the USSR, 1940.
Training, tactics, tenacity
Defence of Stalingrad & Uranus offensive
Planning, Strategy, Tenacity, Surprise
Rommel’s Desert offensives 1941/42
Surprise, tactics
Royal Navy Battle of the Atlantic 1940-43
Tactics, technology
Polish Warsaw uprising
Tenacity/bravery.
RAF Battle of Britain
Strategy, technology