If you could do only one exercise, it should be the kettlebell swing.
Build the perfect posterior. This single exercise works the shit out of every muscle between the base of your skull and your Achilles Tendon.
It’s practically impossible to do this exercise correctly the first time. If you do not get proper eduction and/or instruction you WILL take a serious risk damaging your back.
Here’s a helpful video that Tim Ferriss produced to accompany the 4-Hour-Body