RE: Maximum Weight/Fat Loss
The best cardio is the one which you enjoy (or at least don't hate) doing. Whether that be a treadmill, elliptical, outdoor running/cycling, skipping, trampolining or whatever. There are different types of cardio that you may employ for each of the different exercises such as 'steady state' or 'High Intensity Interval' - more details for these can be sought in the cardio forum.
As long as you are consistently losing a sensible amount of fat per week, then this is the best you can hope for.