If you want to skip the click baity title and just know what the secret exercise is...there is none. Go home. Effort and consistency are the only things that matter.
If you want more Metal Gear Solid amount of insight, read on.
The Actual Article
I draw weight lifting questions like shit draws flies. Poop never asked be a meal. Why poop gotta be a meal? Huh? HUH?
Thankfully, these questions are limited to when I'm lifting weighs (unlike my friend Keenan, who gets them out in public by literally any dude he has a conversation with), but the conversation is always the same.
1. Ask if I'm a trainer (no)
2. Tell me about how they used to be the strongest man in the world (cool story brah)
3. Ask for the secret exercise
The third question in particular always gets to me. I've actually choked up a little. Here is someone who has found themselves to be too fat or too weak for their liking, and have come to the gym to level up. Good on them. But asking for the secret exercise that makes you lose weight, run faster, and fuck harder is belittling the effort needed to actually be good at something. It's saying you aren't interested in suffering through the journey, you just want your reward, and you want it now. It's entitled, and it's ignorant.
The only things that work are consistent practice and effort. In general, this is going to include getting better (see: increasing reps and weight over time) at compound exercises. Which ones you choose doesn't really matter if you are looking for general health. Sure, some exercises will swing the needle a tiny bit one way or the other. I generally find I feel slightly more athletic when I'm training with oly lifts consistently, but in the long term my exercise choice means absolute dick. What is important is that I strive to improve. What matters the most is that I show up and try.
There is no secret. Go forth and get stronger.