[Kragh]

In all of these scenarios im assuming that the animal has taken it on to EAT you, you and the animal are engaged in close quaters combat...Im also assuming that the enviroments would be what you would expect( i.e. broken branches in the fores, rocks by a riverbed etc) but no weapons laying around:

Shark-

Out of all of them I think that the shark in the water would be the most dangerous, being that is in its natural environment, is more agile and quicker than you. Also, if you try to grab the shark your flesh will be lacerated by its skin. One bite will place you into shock and the blood will attract even more sharks. A human being would stand no chance due to our clumsiness in water, you couldnt even punch it due to water resistance...against a shark a human has zero chance unless the shark decides to swim away.

Bear-

The bear would be next on my list due its shear size and weight. The bear would most likely be too tall to punch in the face or gouge the eyes and its fur is so think that any other attempts to stun the animal would be nullified. The bears paws and force of its swing are enough to break bone and absolutely knock you off your feet and stun you, thats not counting its claws which could rend flesh open easily. Once the bear has knocked you down its shear weight would immobilize you and after that there is no chance. If you are still conscience, you might be able to reach for a nearby rock or branch and since its the only chance to get close to its head you could try to gouge eyes and nose...but your most likely dead already.

Lion-

Although Lions are fairly heavy, thier main attributes are speed, agility and claws. I would say that a very strong person could survive a lion attack, but would be clawed nearly to death. The lion is going to spring at you and dig its claws into you, next it will attempt to bite deep enough to sever a major artery and then hold on untill enough blood has leaked out and you are weak and fatigued...after that it has its way with you. Your only chance after the lion has leaped at you is to grab its front arms and using all of your strength try to bend them out. The back claws will be shredding you to death and its jaws will be snapping at you, but a good headbutt might deter the jaw...once the lion is somewhat weakened and the front arms are spread youd have to roll on top of it and put your weight on it. At that point youd have to use your teeth to try to rip anything you can apart. Eventually the lion will tire, if you dont first and once big cats realize they have lost they will run away...they have alot of pride and can be embarrassed easily. They mainly want an easy kill and if thier prey resists too much, they cut there losses and run.

Croc-

Id say this would be the easiest, if you survive the intial jaw crunch unscathed, then just straddle its back and either start gouging eyes or lifting up on jaws...sure not everyone can do that, but out of all of the animals, its the most likely to be survived...running also sounds like a good idea as the croc has no interst in being out of the water. I mean you still have little chance of survival because the initial attack will take most people out immediately.