The title of the thread is obviously tongue-in-cheek since there's probably no single movement that is the be-all end-all of strength or conditioning, but if there were one my vote would probably be for the overhead squat. If you're not doing them, and have access to the equipment, consider adding them into your workouts. Fantastic for full body strength, core stability, shoulder strength, and linking up lots of those muscle groups that lots of folks seem to want to isolate.
Here's a seminal online article by Dan John about the overhead squat and his exposure to it. Depending on your body composition and fitness level, his standard of 15 reps (presumably unbroken) at bodyweight is no mean feat. I can't do it now, and I've been overhead squatting for a while.
Nonetheless, if you're looking to add any new exercises in the new year or for any other reason, seriously consider the overhead squat.
And if you don't have access to barbells, consider doing them one-handed with dumbbells or kettlebells.