What is the most potent stimulant using the smallest possible dose?

In an uneducated guess, I would say crack or adrenaline. But it depends on whether you mean potent by onset time, effects, or duration. Methamphetamines are high up there too.

which is the actual substance your body makes in the “fight or flight” reaction. It is also a neurotransmitter. Drugs such as amphetamines and cocaine, (and caffeine, for that matter) cause neurons to release more epinephrine, and/or inhibit re-uptake (allow it to work longer than usual.) This is quite an over-simplification, but those who are truly interested in the complexities of neurotransmitter and drug interactions will find other sources of information anyway.

The answer to your question partly depends on what you mean by “potent.” Epinephrine has both a lower effective dosage and a much lower LD50 than the stimulants I checked (amphetamines), meaning it takes much less of the stuff to kill you. It also appears to have little or no abuse potential, most people preferring to avoid the sensations of pounding heart, heavy sweating, anxiety, and so on.