I'd look at modern SOF (special operation force) guys. I don't usually like to discuss stuff like this, but as someone who's in the process of becoming a part the US special operations community, to even become an SOF "soldier" (general) requires an amount of mental toughness that I don't think will ever be historically matched.
Gurkhas are some pretty hardcore motherfuckers, as well, I'll say.
To be specific.
They have no home to return to and have accepted death as their fate.
They have beat the russians on many ocassions with small daring terrorist actions.
They were in Afghanistan too.
Maori of New Zealand.
Gurkhas of Nepal.
The Jedburghs. They're the guys that 'cleared' the way in advance of the airborne troops on D-Day. Operating in 3's behind enemy lines, they thought of it was a good time.
Cave-men, duh. All evidence suggests primitive man was athletically far superior to post-settlemment man in every way. Fossilised footprints suggest he was faster than Usain Bolt, arm bones suggest he was extremely strong. Most of the difficulties faced by modern or historical soldiers could be faced by him every day.
After that, I would pick people who still had physically demanding and frequently violent lifestlyes for long periods of their lives, so I'd probably go with vikings or red indians.
I'm a fan of the Three Kingdoms era of Chinese history. I've read the historical and fictional accounts, played the games, etc...
These two generals I think had the best leadership and fighting prowess of their time:
Guan Yu (Still considered a god of war)
Zhang Liao (Led a group of 800 men in an attack of an approaching army of at least 10,000. He killed enemy officers, charged the commander and the enemy was routed. He was in charge of 7,000 total garrisoned -with a large reinforcement army coming- against an approaching enemy of 100,000. He was given an order that was roughly translated to "bloody the enemies nose". As Guan Yu is still considered a god war, he considered Zhang Liao to be his equal as a warrior.)
More brutish warriors, great at fighting but less effective against tactics
Lu Bu (perhaps the ultimate brute warrior, but completely failed in leadership and tactics)