I was intrigued by the simplicity of the photos, the expressions he caught from the subjects, the lighting he used, and the fact that it was an 8x10 Large Format camera.
There were lots of photos fighting for my #1, but the winner ended up being Earnest Hemmingway.
What stands out right away, is the lighting.
From what I can tell, Karsh lit Hemmingway mostly from above. With that, one light camera left and slightly behind the subject, and possibly one light camera right slightly behind the subject.
Both of these side lights (if there are two) form a rim of light around Hemmingway, causing him to pop from the black background.
The light above Hemmingway creates a far more dramatic image than a light, lets say, head on, as it emphasizes the large round collar of the sweater, as well as the shadow of the beard.
This, together with the simple, center framing catches my eye right away.
As for Dark Room techniques, all I can make out from this particular image is the way he would've dodge and burnt it.
If you look closely between his shoulders and the background, you'll see a very feathered grey zone (more dominant on Hemmingway's left side).
I could say that this was done by a third or fourth light aiming at the background, but the feathered grey area follows his sweater curves, where a light wouldn't do that.
So, using Dodging and Burning, I feel he would've done nothing more than dodge in Hemmingway, and Burn out the Background.
Lastly, I'm not aware of the contrast coming out of an 8x10 negative, but it looks like he would've added contrast as well.