The Spetsnaz is pretty hardcore, but I think I'm going to agree with their ranking of the SAS at #1. Does anyone else deserve an honorable mention?
Whereas most of the training regimens of militaries around the world are designed to teach, Russia’s Special Purpose Regiments, equivalent to the U. S. Green Berets, endure punishment daily throughout their training. They may quit anytime they want. The Spetsnaz want only the best, pain-hardened, battle-loving killers.
They spar with the express goal of injuring each other, breaking ribs, fingers, vertebrae, healing only long enough to get back on their feet and complete the training. They are typically deployed for reconnaissance or house-to-house close quarters combat, but are also employed as extremely formidable bodyguards for high-ranking politicians.
They claim that they are not taught to ignore pain, since that is impossible. They are instead taught to enjoy it.