The more you move the more calories you burn. So an intense constant run would burn up the most. Having said that be careful not to overdue and injure yourself.
At a "hard" intensity the results were:
Treadmill - 700 Calories per hour Stair Machine - 625 Calories per hour Rowing - 600 Calories per hour Skiing - 600 Calories per hour Stationary Bike - 500...
Brisk walking, rowing, running and cross training are the best exercises. For an overweight person I would recommend the cross-trainer as there is less impact and they will find it easier. Otherwise...