As a twenty-something, you can push your body harder than you can later. For this decade, I recommend Clean & Jerk. It’s a full-body exercise that works the glutes, hamstrings, quadriceps, deltoids, triceps, biceps and core muscles. It also helps build power and overall strength.
Learning proper form in your 20s will help you through all the decades to come, and this exercise works multiple muscle groups and requires excellent form with each step.