Whichever activity the individual is least likely to have given up within 6 months. 
I'm pretty sure running is up there for "no. of calories burned per hour", but equally I think we can sometimes take for granted that it's a pretty difficult activity to get into, especially for someone who is unfit and/or overweight. Plus the higher propensity for injury compared to most other activities. My own experience is that I didn't even want to consider running until I'd reached a certain weight and level of fitness (partly based on the classic misapprehension that "running f*cks up your knees", but that's beside the point!...) but I was happy to ride a bike and/or attend a gym 4/5 times a week over the course of 6 months, mixing up weight, spinning, other cardio...
It may well be that the person tries out a few different activities, and that one of them becomes a dominant one which becomes more fun/more likely to be persevered with, because (a) they're good at it, (b) they decide to take it up a little more seriously, so that "exercise" becomes "training", (c) they join a club, (d) spend a ridiculous amount of money on a bike/pair of skis/Chinese Wall marathon entry, etc... or all of the above.
Hopefully you can see where I'm going with this. If someone asked me what's the best single exercise for weight loss, I'd try to get them to frame the question differently, suggest a mixed approach to exercise involving cardio and strength training, tell them that they're only going to acheive long term sustainable weight loss by sticking with it and being consistent, and wish them good luck.