A recent study said running is the best exercise.
The intensity in proper running is doing wind sprints at the end. Run as hard as you can the last 5 minutes.
There is no “best exercise” for everyone, especially as people get older. If you have been running for years it probably is the best exercise for you. But because it is a “high impact” exercise, running causes all sorts of injuries in some people. In others it improves their bone density and doesn't cause many joint issues. But this depends largely on their physical makeup and previous exercise experience.
Based on people that I know... running seems to be good for younger people who are not overweight. Old fat people are frequently just torturing themselves and often end up with injuries. Most of them would be better off with something that is low impact like walking, cycling, rowing, or swimming at least until they lose some weight and make some sort of progress on their physical fitness level.