Comment by noelwelsh
I am someone who enjoys exercise, but I only enjoy certain types of exercise. I cannot stand mindless "monostructural" endurance events like running or swimming. However I'll happily chase a ball until I am exhausted. I need some complexity and mental stimulation in my exercise. My favourite type of exercise, which I only discovered relatively late in life, is what I call "compositional movement", where the movement complexity is the point. Things like gymnastics, dance (particularly breakdance), some kinds of parkour, and so on. In other words, I think different kinds of exercise suit different kinds of personalities. It's unfortunate that most physical education doesn't do a good job of introducing people to a range of movement possibilities.
This is really where the focus needs to be for young people, and I think it's where our education system is really failing.
What exercise is best? Why... the one you enjoy and will continue doing!
Focusing on ROI can be a good way to view it for the right personality type, but I think that mentality can be harmful for the wrong personality type who will just grind out activities they don't really enjoy until they give up in exasperation.
I suspect for most people the trick is just finding something - anything - that is physically demanding that they enjoy, and then sticking to that.