Couple of things. First thanks, because- while not a strength coach, I have spent 47 of my 68 years in the gym,and I get asked the same question. You could just as easily answer, "it depends". Because, well. It does depend. I recently did a piece on how powerful the all-too-common but still bitch hard pushup is to overall health. Like a burpee (which I hate, but that's because of a busted toe), there are exercises which are so good because they work us all over. Like the Turkish get up. But there is No One Thing. To that, perhaps, I would agree with Gary: just do something. Christ, we're a nation of couch potatoes or extremists wanting One Big Answer. There isn't one. I could sum it up and include your point: Do the work. Track the work. Applaud the work. Take it up a notch. Rinse, repeat. When you plateau, shift things around. Doesn't matter if I run or lift or ride or kayak. The gym has been my second home my entire adult life. There's a reason I have this body at 68. I started and didn't quit. That alone is a One Big Thing.