Running is easily the best cardio, you can moderate your pace and push yourself to the absolute limit safely. More than swimming, biking, rolling, hitting the bags, more than anything. You can stay at your max heart rate for longer than in any other activity. Yeah you can hit the bag for an extended period but your shoulders will give out before your cardio ever does.
However, it's not for everyone. Some people's feet cue at the wrong angle and long distance running simply isn't an option as every landing puts undue torsion on the knees and hips.