The actual amount you need to exercise, or the intensity, is not very big. The good thing with exercise is also that once you make it a habit to walk, say, 20 minutes a day, then walking 25 minutes a day becomes pretty trivial, and so on.
In my opinion the best measure of exercise is perceived effort. So while you're asking for objective answers, I think a lot of this is inherently subjective.
The benchmark you're asking for is also ill-defined. For example: How frequently to be as effective as what type and what frequency of therapy?
Based on my own recent experience I would suggest the "dog".
Edit: A motorcycle that is, unless someone has created 200mph dogs...