Rambo. It is now a name synonymous not only with combat, but with one man taking on everyone in sight all on his own. There are four Rambo movies to choose from, but First Blood is the best received by a wide margin (87% on Rotten Tomatoes, compared to the next highest - 2008's Rambo - at 37%), even if First Blood: Part II inspired one of my favourite Pauline Kael quotes when she described it as "like a tank sitting in your lap and firing at you."
Like Rambo, "Dirty" Harry Callahan is a man known for taking care of business. Although technically Harry works with a partner - two, in fact; he manages to go through two partners in one movie - he gets most things done on his own, whether it's foiling a bank robbery (while still eating a hotdog!) or going head to head with a serial killer.
He may not know why he's so proficient at kicking ass, but there's no doubt that if you send Jason Bourne in, he's not going to need any backup. Weapons work? He can do it. High performance driving? He's got it covered. Hand-to-hand combat? You better believe it. Added bonus, not only can he do all that but if he were so inclined to make an action hero quip afterwards he could do it in multiple languages.
There might not be 88 members of the Crazy 88 gang, but the Bride still mows through an impressive number of people in Kill Bill: Volume 1. Hell, she even manages to take one guy out while still confined to a hospital bed. If the Bride is out for revenge, just get out of her way as quickly as possible.
The one man army to end all one man armies. The guy doesn't even need shoes to take down a bunch of terrorists! No contest, John McClane is the first person you should call if a situation is impossible - though he tends to just wanders into these situations instead of actually being called in.