Question

According to Churchill who posed the biggest threat to the Allies?

Answer 1

During WW2, a variety of different threats were before him & problems, mainly during the first years of the war before Operation Barbarossa & the United States declaring war on Japan with Germany responding to it with their own declaration.

Explanation:

When France fell to Germany & it's Axis Allies, they had full dominion over Europe, created a variety of problems for Britain & Churchill as a whole.
America was Neutral during this time.
The Soviet Union was 'allied' to Nazi Germany, remember the pact Hitler & Stalin made when they both invaded Poland.

This left Britain very much alone & in trouble, not to mention the launch of Operation Sea Lion or the planned invasion & bombings of Britain which for saw the Battle Of Britain.
With the Blitz spreading around all the coastal cities of Britain, & the raising of areas of these cities to the ground, & the psychological 'Vengeance Weapons' falling on British soil.

Churchill certainly knew the Axis powers were a threat.

I'm not a supporter of the Nazi regime, but in my opinion, Hitler should have waited till early spring the next year to launch Barbarossa instead of about early to mid June in 1941, that way the mud they got bogged down in the Winter, would have cleared up or at least gotten less worse.
Then they 'Blitzkrieg' up through the Russian lines, going straight for Moscow & then carry on a couple of Kilometres then stop & hold it until the year was up.

During the space between the 'year was up' they should have 'filled' in the gaps, managed their captured areas, prepare for a possibly weak offensive & prepare for their own, but who knows, the Soviets may have fallen due to that strike at them.

This way at least, a large piece of fertile, industrial, usable land instead of tundra & taiga that most of the successfully evacuated Russians would have been forced to work through autumn & winter, which Germany would have been able to secure pretty easily if at that time they realised & built a couple divisions of fast & powerful vehicles for the open landscapes & be able to 'tweak' their forces to be able to work in the Russian environment.

Anyway, enough of all that 'extra' information.
The Nazis were the main threat to the Allies, the Japanese, not to much, but still they were rather cruel too the Allies.

Hope this helps.