Pound for pound, it's hard to find a more bad-ass group of people.
My Krav instructor wen to LA for his teaching instruction. His instructor was an ex-Mossad agent. After a couple of days of being beaten down during training, some of the students complained that they were worn out. After a while, the instructor got pissed. He lifted up his shirt to reveal numerous bullet wounds, knife wounds, and explosive-related wounds He told them that they didn't know what being beaten down was and to shut the hell up.