Ask me for an upper-body workout, and I'll give you one exercise: the standing single-arm overhead press.
Here's why: It works the deltoids, biceps, triceps, lats, and pecs, as well as all of the stabilizing muscles in your entire body. It even works your glutes since you can't have a weak or sagging butt when pressing a large amount of weight overhead with one arm (it just doesn't happen).
And if you do the exercise alongside the squat and deadlift, you have everything you need for a basic total-body training system.
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No other exercise comes close to offering that many advantages.
If you already have the overhead press in your workout, great. But I bet you don't have it programmed like this. It's called the 2-3-5 method, and I use in my book Mass Made Simple.
The rep scheme follows a simple "ladder" formula that allows you to handle a lot of volume. You can follow it no matter what your goal--sports, hypertrophy, fat loss--because the training effects carry over to all of them.
Tip: A kettlebell works better than other training implements for this workout. That's because the bell's weight can shift, making the exercise harder and activating your muscles more than if you had a stable weight. (Check out The Ultimate Kettlebell Workout.)