Just One Exercise by Michael Boyle of t-nation is one mans answer to the question “if you could only do one exercise, what would it be?”
He says push a heavy sled because each step engages one leg and there is massive core muscle recruitment involved with single leg movements. While this is not a single leg exercise, each step of it is so the core activation is superb. It also engages the glutes and hamstrings in the way that you come to expect from glute ham raises or good mornings. Check out the videos in the article if you’re not sure what he’s referring to.
I would have answered differently but for a similar reason. I say one arm corner barbell shoulder press because they recruit the pec, deltoids, triceps and the muscles of the core.
Once I started doing single arm and single leg work, I was able to dramatically reduce the amount of oblique work I was doing. In fact, I haven’t performed a single side bend or side planking movement in months and I’m happier than ever with the way my mid section looks.