Whatever you enjoy doing as if you don't like doing it, you sadly won't do it on a regular basis
If you base it on calories per hour you need to consider whole body exercises such as cross country skiing, running and rowing.
If you base it on what you can potentially do for longest (therefore more calories used) consider going out walking at a steady pace or cycling as these are generally activities we can do for extended periods of time.
You can integrate many other things into your fat burning regime. Swap regular tea for green tea (100 extra calories burnt daily) walk/ cycle instead of driving short distances. Take the stairs instead of the elevator and consider what you're eating
I hope that is of assistance for you. CF