Caffeine.
Look, I've argued that knee surgery, eyeglasses, etc, etc are performance enhancers. Because, quite frankly, they are. If they make you runner better, see better or, basically, do anything better, they enhance your performance.
So, as I mentioned earlier, if MLB isn't testing for it, you're only cheating yourself if you don't find a way to do your best. But, once MLB decides something is illegal AND specifically disallows it, it's cheating baseball.
Were amphetamines tested when players were gulping them by the handful?