Disjointed Post Alert
There seems to be very little agreement on the internets on this one, but these are listed very consistently at the top, in no particular order:
I told myself before searching that it would be either Swimming or XC Skiing.
One interesting aspect of your question is that you specified "pro". Obviously, the better you are at something, the fewer calories you burn doing it.
To answer the OP, I will say:
3. Pro Soccer (they get to stand around sometimes)
2. Cyclists (they get to coast a little, but even when going full bore, they "only" use half of the body)*
1. Marathoners (use the entire body; never get "coasting breaks")
*Yes, I am aware that Pro cyclists have one of the highest average LTs in all of sports, second to only XC Skiing, if I remember...