You know, start with recruits who would already be special forces by real-life standards, and dump them into a hell with a 90%+ mortality rate, and the survivors only make it by killing and eating their comrades. Where the deliberate goal is to produce a dehumanized killing machine.
That’s how the Sardaukar in Frank Herbert’s Dune universe were trained - and they started in friggin’ infancy…
Dorsai.
And the Fremen showed them who’s boss.
I imagine the Imperial Space Marinesrank highly on the list.
Probably not comparable to the Sardaukar, but the soldiers in Joe Haldeman’s The Forever War had it pretty rough, with the added difficulties of much more hostile planetary environments, arguably poor command and control, and being deployed to combat in ways that made the Starship Troopers movie look like Gaugamela.
While the Space Marines were pretty tough, the Catachan Jungle Fighters were probably far better trained.
Describing the two:
Space Marines are given lots of augmentations and power armor, are effectively immortal (let’s just say they are known to live for eons… and still don’t die of old age). They often have centuries of training in frozen ice-worlds, desert wastes, or dead space before encountering serious combat, and are expected to be able to use almost any weapon handed to them under any conditions.
The Catachan have it worse. They are born on a hellish world so filled with killer creatures that they suffer a ridiculous mortality rate. Every creature big enough to see is aggressive, vicious, carnivorous or omnivorous and eager to eat you, and most are toxic. The plants are even worse, and the insect life is more dangerous than the killer animals. The people there learn to use knives before they can walk, and frequently look upon service with the Imperial guard in suicide squads as a step up in life. They don’t have power armor, just a flashlight and a t-shirt, ad they can quite effectively take down demons, tanks, and so forth.
The Unsullied in the Song of Ice and Fire’s city state of Astapor are pretty tough. These are eunuch slave soldiers, two thirds of whom die in training, who are stripped of their individuality, given drugs to strip them of their sense of pain, and taught unconditional obedience to their owners.
What about the berserkers with the ear grubs or whatever they were from Beastmaster?
Try becoming the Master of Sinanju.
I recall an old novel, Son of the Morning Star ( I think was the name ) where the protagonist used a method of training that involved being sent to a Valhalla-like alternate dimension for ten years of fighting; he came back with his emotions burned out from the sheer trauma of the constant combat and exposure to Lovecraftian monsters.
The inhabitants of Pyrrus from Deathworld are taught survival literally from birth. The planet is nasty enough that returning Pyrrans are loaded into armored pods and trundled to retraining centers because being off planet for any time means they are dangerously behind in their training. And that’s just to deal with what gets past the city’s automated defenses.
In the Elvenbane series, the elves “train” their human slave-warriors by having them fight each other until one is crippled or dead. Note that this doesn’t generally produce very good fighters, just lots of crippled and dead ones.
They’re also given a puppy at age 2 and told to kill it at age 6.
Coming at the end of the list up to this point, this made me burst into laughter. 
doesn’t have any contributions
At the risk of prompting the obvious joke, Bruce Wayne would have to be somewhere on that list… especially since his training also involved a great deal of non-physical education.
I seem to recall that Granny Goodness went through something similar. Her female furies are raised to be fierce warriors as are other members of Darkseid’s troops.
Terrible film, but the soldiers from 1998’s *Soldier *starring Kurt Russell went through a true survival of the fittest boot camp since childhood. I remember a scene where a fat kid lagged behind on a run and was summarily killed.
If I remember right, Shakir’a’s people in the Second Milenium series were pretty brutal [Snowbrother, Shadow’s Son, etc].
I’ll toss in the Patryns from the Death Gate Cycle. They’re all imprisoned in a magical, apparently sentient correctional facility gone rogue, bent on killing them. It’s at least as much a magical training as physical, but still, pretty damn tough.
Maybe not the pinnacle, but an interesting example of prison-world in fantasy.
There’s Stroggification from the Quake series; the cutscene from Quake IV involves lots of brutal surgery with no anesthetic. Spikes, saws and so on gouging away.
Sounds like Doctor Who’s Cybermen.