Most of the sources that attribute a particular strength or fear towards allied troops typically come from the allied country perpetuating the claim. The American press tends to exaggerate or glorify American contributions to the war, the British press tends to the do same... and Canada is no different. You'll notice that all of the news or media sources, usually around this time of year, tend to reiterate harrowing tales of military bravery, and that the enemy particularly feared "us".

If you're interested in this more I recommend these sources:

"The German Army at Vimy Ridge" Hayes, G. 2007

"Constructing Memory: The Vimy Memorial" Pierce, J. 1992

"What Jerry Thinks of Us... and Himself" from Intelligence Bulletin, Dec. 1944 (WW2 opinions).

I actually got those sources from another reddit post, analyzed them, and agree that they are good sources.

This is not to discount Canadian contributions to the allied cause in WWI. They played a very significant role particularly within the last 100 days of the War. Sir Arthur Currie was also an extremely accomplished general whom David Lloyd George actually considered championing for Supreme Allied Command. It's just that, on the front line, Germans saw Canadians as identical to British troops. This is mostly because Canadian uniforms were similar to British uniforms, and usually Canadian regiments fought alongside British other Imperial units (from Australia, New Zealand, South Africa, etc).

There's little evidence to suggest that German or other Central Powers' troops thought anything unusually fearsome about Canadian soldiers. They thought of them as the same as British soldiers, and therefore through word of mouth gave them the same reputations.

I realize I'm a little late to the party but I'll write something up anyway because I have in the past and I've looked into this topic fairly extensively.

So this thing about the Canadian reputation in WW1 pops up pretty regularly, quite frequently in relation to the battle of Vimy Ridge. Whenever the question of Canadian reputation arises the first thing we must ask is "their reputation where?". Like /u/I_like_Triscuits I have never seen a German source indicating that they thought anything special at all about Canadians or troops from any other dominion in WW1. What references to Canadian troops there are tends to be minor anecdotes and even then it's just as another kind of Tommies from another part of the British Empire.

"But Vimy Ridge..."

But nothing. Vimy Ridge was important only to Canada and really only after the war as part of the founding myth of the nation. Australia had Gallipoli, Canada had Vimy Ridge. The Germans did appoint a commission to look into the battle of Vimy Ridge, not because they were particularly afraid of Canadian troops or because it was a particularly serious blow (in fact German High command did not consider Vimy ridge anthing more than a stalemate since no breakthrough was achieved), they merely wanted to know why the defenses had not held. The reports conclude nothing at all about the fighting qualities of the Canadian Corps but merely states that frontline commanders ignored repeated warnings of an imminent attack and that reserves were too far back to relieve the numerous strongpoints on Vimy Ridge in time. That's it.

"But trench raiders and shock troops..."

Yes yes, the Canadian Corps were used by the allies as shock troops and they did a lot of trench raiding. Everyone did trench raiding. Again, the Germans were not unaware of the Canadian Corps. Quite the opposite, they kept tabs on it whereever it moved and generally rushed reinforcements to the front wherever the Canadian Corps went but this was not because they were afraid of the Canadian soldiers but because they were well aware the Canadians operated as a cohesive unit meaning that if you saw one Canadian soldier you could more or less assume there were 99 999 more around and 100 000 troops arriving on the line anywhere was cause for concern regardless of their nationality.

Based on all available evidence we can pretty safely conclude that the Germans thought nothing special of Canadian troops as such. Neither in terms of fighting quality or ruthlessness.

Among the allies there is a bit of a different story but I think /u/jonewer summarized it pretty well in this thread which is alone the one I assume /u/I_like_Triscuits is alluding to in his post.