In reply to The Lemming:
I think it's an interesting question to make one think about exercise theory. But I can't think of any situation where or why one would be limited to just one exercise. So I'd assumed it was more for forum banter than practical application (which is totally fine btw). If that's not right then it would be good to know and interesting to hear why just one exercise.
Back to the question: in modern sports theory there is the idea that the best exercise is the one that is most beneficial to the sport being trained (functional training). So I suppose it depends what the purpose is. Is this for climbing, or bodybuilding, general fitness or something else?
For instance if it was climbing I'd probably go with fingertip pull ups but for bodybuilding the clean and jerk/snatch or even deadlift might be better.
Then again you might want to invent a complex compound movement that incorporates as many different muscle groups as possible. Multi-planar training is popular at the moment. This is training with exercises that involve movement in 2 or more of the 3 movement planes.