Saw another one of these the other day. I don't even remember where.
You know the formula:
If you were trapped on a desert island and could only bring a single books/piece of training equipment/do one exercise...what would you do?!
There seems to be a societal obsession with the question, and I really don't get it. Is there an epidemic of people being stranded on desert islands that I'm not aware of? If I am stranded on a desert island, am I really going to be worried about the fact that I don't have my copy of the Lord of the Rings, or my trusty kettlebell to swing around (frankly, if I'm trapped on a desert island, I don't think I'm going to be real worried about whether or not I'm getting enough exercise).
I get that, at its core, the question is asking "what things are important to you" or, in the case of fitness, "what tools or exercises are most useful to you", but why not just ask that? And really, why ask at all, unless you'll do something with the information.
Joking aside, the Lord of the Rings really is my answer to the book question (and yes, it is one book, don't start with me). But that doesn't mean I'm just going to sit around and read the Lord of the Rings and nothing else for the rest of my life.
I love kettlebells (although, with the SFL coming up, barbell work has taken up more of my time of late), but I'm (clearly) not going to stop lifting barbells, doing bodyweight exercises, or whatever else. Why should I? There is no earthly reason.
Honestly, the question makes the least sense to me when applied to exercise. I could see devoting some serious time to really diving deeply into a book, but what world do you live in where you're only allowed to do a single exercise? Unless you have time for exactly one rep per day, in which case I would humbly suggest that the exercise you need to perform is organizing yourself better.
I'm all for "hacking away at the inessentials" (to borrow a Bruce Lee quote), but why not do it in a way that actually makes sense? Unless you know you're about to be stranded on a desert island, why the fuss?
(If you know you're about to be stranded on a desert island, I suggest the most valuable tool for you would be a radio or other device that helps people find you and you off the friggin' island.)