Native Americans as a whole, with several clear standouts; Cheyenne Dog Soldiers, the Lakota, the Apache, the Comanche, Seminoles, Iroquois; the list could go on forever.
My vote would always go towards one of the Native American groups. Their tactics were great. Crazy Horse has always been one of my favorite historical figures.
And I'll toss in the Mongols. Unifying various nomadic tribes under a single banner and forming the largest continuous empire very rapidly. Excellent horsemen and archers. The draw weight on their bows were pretty crazy, combine that with riding a horse and you can see how they devastated other militaries at the time. They inspired fear and used that reputation to conquer without having to lift a finger, but those that stood against them suffered greatly (Baghdad).
I see some shows that try to equate different warriors or armor against each other creating
anachronistic situation to try to compare. The thing is that this makes no sense because the conditions that created the units and equipment where simply not the same. One does not call in the Air Force to take and hold a beach head.
I think each in their time that was victorious.
I like the Landsknechts myself.
I would say the Prussians (renowned for their discipline and mobility, often winning despite numerical disadvantage) and the Romans (they always changed their tactics and equipment to suit their enemies, and their heavy infantry was hard as nails).
The presentation of the Spartan lifestyle which Thucydides and Xenophon both discuss is rather remarkable. The level of military influence and organization within Sparta has never really been matched in any nation since; not just in tactical terms but also in how severely the whole military structure pervaded regular society.
I would equate toughness with "the ability to take physical/emotional punishment and still achieve their goal."
I feel like I don't have enough of a commanding view of military history to really make an informed statement on the matter.
If you necessarily talking about military exploits, there are some dang tough people who are not allowed to be talked about on the main forums.
As individuals: the British Commandos of WWII.
As one large group: The Mongols.
Why.... The Sacred Band of Thebes... they even beat yer gosh darn Spartans... but then they lost to the Macedonians.... so... them? But then the Roman's took over that whole thing... how about them? Wait... and so on.
I tend to think of battles or operations, not specific armies. The Spartans at Thermopylae. The Texans at the Alamo. Interestingly enough ... both lost, but held out long enough to do damage. D-Day. Battle of Makara Market in Mogadishu is a hell of a story. Okinawa. Russians in Stalingrad.
There's always the Israeli military. Gotta respect an isolated country, surrounded by enemies, still standing by sheer force of will after 60+ years. Not only that, but they got invaded ... and ended up taking territory from their invaders. Heh. A lesser country, with a lesser military, would've yielded decades ago.
For individuals ... Carlos Hathcock was a wrecking ball with a sniper rifle in Vietnam. Same with Vacily Zaytsev in the Battle of Stalingrad. Yonatan Netanyahu (brother of Israeli PM Benjamin Netanyahu) was a hero of the Yom Kippur war, involved in the hunt for, and assassination of, "Black September" -- the terrorists that killed the Israeli Olympic team in Munich. He was eventually killed in action as a commander of the Sayaret Matkal (Israeli Special Forces) during a successful counterterrorist raid in Entebbe.
George Patton was a legit badass in WWII ... but he originally made his name in the hunt for Pancho Villa, when he (unordered) drove a supply Jeep up to the hideout of a Villa lieutenant, got in a gunfight with three of them (with his ivory-handled six shooter) ... killed them, and drove through Mexico with their corpses strapped to the hood of his Jeep.
For modern warriors ... Navy SEALs, Army SFOD-D, Green Berets, DevGru, British SAS, Israeli Sayeret Matkal, German GSG-9. Lots of badass units out there. Black Ops are in their own class -- smaller incursions, less notoriety, high stakes. Most stuff you'll never know about, but the details that get out are always impressive. Navy SEALs that took down Bin Laden. GSG-9 raid on hijacked Lufthansa flight 181 -- killed three terrorists on a small plane, rescued 86 hostages. Assassination of Columbian terrorist and druglord Pablo Escobar in the 90s -- carried out by Columbian SF with significant assistance from US Army Delta and Navy SEALs.
Still more impressive was the three SEAL snipers that took out three Somali Pirates with three simultaneous head-shots from the bough of one rocking ship to another ... rescued the captain and crew of an American freighter.
I'm sure the really interesting stories are the one's we'll never know, though.