An interesting topic subject to debate. If one assumes that the primary goal of the men fighting a war is to kill their enemy, then those actions responsible for the most death and destruction should be considered most important to the cause. In this light, artillery fire would seem to have been the most important factor in the war. Recce spotters were the only aviators directly associated with this activity. The extent of their usefullness in this action is beyond my knowledge and is probably also subject to debate.
Not certain of exact figures, but if 8 million died in the war, I would guess than roughly 100,000 or so died directly at the hand of fighter and bomber crews. If even close to accurate, the effect of such action on the outcome of the war was not significant. To their credit as seen in future wars, both sides visualized and utilized strategic bombing, but lack of quantity and technology minimized its effectiveness in the first** war.