I think figure 9.3 is the most relevant figure. The majority of the chapter discusses the various regimes of stretching represented in this figure. I also think it is important because it illustrates behaviours which, at least I, wouldn't have thought of intuitively.
I like figure 9.10 as it compares two molecules that essentially perform the same function - binding O2, however you can tell by the plot that the way in which they bind is very different. The myoglobin curve represents the binding of 1 O2 molecule while the haemoglobin curve represents binding of ~3. This is one of the first experiments you should do with enzymes and substrates.
I'm quite fond of a few of the figures. 9.4 is good in showing the effects of extending our DNA, 9.7 brings us back to alpha helix formation, 9.9 demonstrates the effects of cooperativity, and 9.10 is good for showing allostery