ORIGINAL: Bodge
There seems no doubt that the rudder requires the largest force, but I always put my best servo(s) on the elevator. If I were to lose control of any surface, I would rather keep the elevator...
Amen... I can get a plane down without a rudder and on one aileron, but lose the elevators and it's all done...
In order of most torque / speed, I vote for rudder, elevators, ailerons...
Having said that, with the choice of servos available today, I usually use Hitec 5955's on all surfaces on 30% and larger airframes... Close in price to the other top of the line servos, programmable, no gear slop after 75 flights and strong as hell...