I would call Tamerlang (aka Timur, Timur-aksa). I think, he was the best. He was a real warrior and leader.

did he go against Soviet union?

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Funny Of course he did, Soviet Union was very strong in XIV century (when Tamerlang lived) it even possessed nuclear spears and longbows with self-aimed arrows and ancient Soviet warriors used cell-pigeon-phones and cevlar armour
Do you realize how cool was Tamerlang???!!!!

In the West we know him as Tamerlane.

http://www.carpenoctem.tv/military/tamerlane.html

He was born in the 14th Century, and reunited much of the Mongol empire.

By all accounts a remarkable military leader.

Hm... Mannerheim? That guy who was general of Russian empire? Whos only achievment was that he won few battles with newly-formed, not-trained Red Army? And that guy who built famous line of Mannerheim, which was easily destroyed by Soviet army in 1940?
His only achievment was giving Finland independense from Russia. And of course, he was first leader of modern Finland. But I think that he was rather good politician then good military leader... There many-many people realy good compared to Mannerheim: Zhukov, Suvorov, Vallenstein, Fridriech Barbarossa, Napoleon, Alexander the Great, Chingis-khan, Tamerlang, Akbar, Patton, admiral Nelson and others.
Mannerheim is just one of thousands generals who are not remarkable at all.
I understand that he is national hero of Finland, but as military leader...
You made me laugh