Probably the least effective health behavior change strategy is:

The answer is changing several behaviors at once.

We're all born with different genetics. In order to know whether a certain health behavior is effective, we need to try it out and see if the behavior actually made positive impacts in our life.

if we change several behaviors at once, we wouldn't be able to monitor which behaviors which are working for us.

Health behavior has a great contribution to the wellness in the people. So the least effective health behavior change is changing several behaviors at once. This change in behavior sometimes can only happen through outside factors. Changing the environment around you and involving the people around you will have change in behavior happen.