By far the most famous was Winston Churchill, who gave speech after speech in the run-up to World War II warning of the threat the Nazis posed to the world.

Churchill on November 23rd, 1932, two months before Hitler's rise to power: "All these bands of sturdy Teutonic youths, marching along the streets and roads of Germany, with the light in their eyes of desire to suffer for their Fatherland, are not looking for status. They are looking for weapons, and, when they have the weapons, believe me they will then ask for the return, the restoration of lost territories and lost colonies."

Churchill on March 23rd, 1933, two months after Hitler's rise to power: "Thank God for the French Army."

Churchill on November 28th, 1934: "Germany is rearming. That is the great new fact which rivets the attention of every country in Europe, indeed in the world, and which throws almost all other issues into the background."

Churchill on the Munich Agreement, October 5th, 1938: "...we have sustained a total and unmitigated defeat...You will find that in a period of time which may be measured by years, but may be measured only by months, Czechoslovakia will be engulfed in the Nazi regime...I fear we shall find that we have deeply compromised, and perhaps fatally endangered, the safety and even the independence of Great Britain and France."

Thankfully, his country promoted him in its hour of need.

This might be slightly outside the scope of your question, but Jacques Bainville wrote a book in 1920 called "The Political Consequences of Peace" in which he predicted the following:

• The collapse of trust between the former Entente members as each member would go back to its former ways.
• The fragility of a peace & new european order based on Wilson 14 points without serious commitment of the USA in the League of Nation.
• The collapse of the Weimar Republic and the eventual rise of a "providential man" capable of reigniting the flame of Prussian Militarism, which had merely been forced to "hibernate" in a society frustrated by a defeat its own population did not believe in.
• The utilisation by a remilitarized Germany of the "peoples right to self-determination" as a casus belly or powerful tool of political pressure against Czechoslovakia, Austria, Poland and the western allies.
• The Anschluss.
• The incapacity of France & Britain to align their strategic goals when it comes to preserving peace in Europe.
• The erroneousness of Keynes economical approach to his analysis of the post-WW1 balance of power.
• The inevitability of a soviet-german treaty as both were diplomatic pariahs and had a common historical enemy in the form of Poland.
• A German-Polish War with Danzig as trigger.

Jacques Bainville had a large influence in France, especially amongst conservative circles, but his work was more or less completely overshadowed by Keynes carthaginian peace theories on the international stage during the interwar era.

He died in 1936, to early to witness the "prophetic" nature of his work, but others did " reap the benefits". When Pétain came to power in 1940, the nationalist & conservative elements that had for two decades denounced appeasement & disarmament policies were nearly untouchable politically, until their own political shortsightedness and gross misunderstanding of the true intents of Hitler came to light.

You can read his book translated in English over here.