Let's start with the historical head to head match-ups.
In Sept 1939... Soviet crush Japanese.
Yes let's
Lake Chasan July 29th - August 11th
KIA:
Soviet: 717
Japan: 526
WIA
Soviet: 2,752
Japan: 913
Battle of Khalkin Gol May 11th - September 16th
KIA:
Soviet: 7,974
Japan: 8,440
WIA
Soviet: 15,251
Japan: 8,766
Not much of a 'crush' if I say so, looks more like a draw to me, and that was with nummerical superiority and what is considered the best Red Army commander of the time,
Georgy Konstantinovich Zhukov.
So your argument falls flat at that point already.
In 1940... France beat Italians while losing to the Germans.
So Soviets and France win over "worst" rivals and are eliminated from our choices.
Yes, 'beating' an enemy while defending the best defensive terrain possible: the alps. It doesn't really give any idea whatsoever of either armies respective capabilities, not to mention that fighting was to short to draw any kinds of conclusion other then: defend in mountains, it's good.
So again your argument falls flat
Worst 1940 Army down to Japan's and Italy's.
Japan at least holding its own against China in 1940. Not anything to write home but compared to the Italians in 1940.
Sino-Japanese War 1937-45
Losses:
KIA
Japan: 388,605
China: 1,320,000-2,000,000
WIA
Japan: 520,000
China: 1,797,000-2,200,000
'Held their own'? I'd say they were defeating the KMT and Communist armies most of the time. With a casualty ration of between 4:1 and 5:1. So again your argument falls flat.
The COMPASS for worst in 1940 points only to the Italians.
Operation Compass
36,000 Commonwealth forces vs. 150,000 Italians
500 vs. 3,000 KIA
115,000 Italian PoW
If one simply looks at compass for an estimation of the Italian army, yes it might be considered the worst.
But I'm a bit more interested in taking a much closer look at the Red Army,
so let's take a closer inspection of the Red Army's performance during the Winter War.
Tolvajärvi December 12th 1939
KIA
Soviet: ~5,000
Finns: 100+
WIA
Soviet: ~5,000
Finns: 250
Soumussalami December 7th 1939 - January 8th 1940
KIA
Soviet: 13-27,500 (including MIA)
Finns: 1,000+1,000 WIA
Kelja December 25th - 27th 1939
KIA
Soviet: ~2,000
Finns: 141
WIA
Finns: 375
Raate Road January 1st - 7th 1949
KIA
Soviet: 7-9,000
Finns: 402
Kollaa December 7th 1939 - Marh 13th 1940
KIA+WIA
Soviet: ~8,000
Finns: ~1,500
Total Winter War:
KIA+MIA
Soviet: 126,875
Finns: 25,904
WIA
Soviet: 188,671
Finns: 45,557
PoW
Soviet: 5,572
Finns: ~1,000
The Soviet had a casualty ratio of 1:5 against a peasant militia army.
So the Italian lost in a campaign an enormous amount of troops to a well-trained, well equipped, well-supplied enemy with surprise while themselves being badly led, wrongly trained, badly equipped and badly supplied.
On the other hand the soviets lost 5:1 while well supplied and well equipped against a enemy with hardly any equipment and woefully under supplied.
I find it enormously amusing that some people on this board gives an tremendous amount of excuses to the Red Army, while utterly condemning the Italian one for roughly the same factors.
While the Italian army did bad, the Red Army did worse, far, far worse.