If you wanted to add one accessory movement, that would be helpful for golf, to a workout routine what would it be?

I am looking to add some accessory work to my workout routine and was looking for some of the top exercises to add. I am doing all major movements and have been working out for 18 months 5 days per week(Push, Pull, Legs, Upper, Lower). I also do Yoga 3-4 times a week and am overspeed training with Super Speed.

I was thinking something like russian twists, kettlebell swings, or some sort of med ball throw. What would you add?