Lion. Over the years, a few have been killed bare handed or with things like pocket knives, etc..... Albeit, with bad injuries sustained. Also a few have been run if the person put up a good fight.
Second choice would be shark, with the caveat being that it would have to be close to shore. People have survived bites, again with massive injures, and been able to paddle enough inland that the shark didn't pursue.
Crocodile scares me the most, fast in water, fast out. You've got the tail, the mouth, and the rolling action to deal with, plus they're generally a more surly beast than the American alligator.
Yeah, Crocodile wins this.