score:6
It's probably alcohol, with tobacco (nicotine) and caffeine at distant second and third.
These drugs don't directly enhance performance, but are great at maintaining morale. In the case of alcohol, although it impairs performance and is easily abused, can also increase courage - see the term Dutch Courage. Many historical militaries also practiced a "last drink" ritual before engaging in particularly risky battles, like Japan's bushi-nin. See also: https://skeptics.stackexchange.com/q/25250
For evidence that these drugs were widely used, look no further than soldiers' rations:
Upvote:2
The natural drug epinephrine (adrenaline).