There isn't just one exercise that is good for teens. However, I think that any exercise that gets your teen moving and doing something productive that gets them off their phone and being more involved in real life, not technology
Treadmills or walks are really good places to start since the increase your stamina. If you are near one, up mountain hikes can also increase stamina. Once your stamina is well enough, you can move on to strength oriented exercises, such as push-ups and curl-ups. The fabulous 7 minute workout is great for this.