Whats the minimum voltage and current to feel stimulation on skin?
This is from JG Webster, Medical Instruementation, Wiley and Sons

The 50th percentile threshold for men is 1.09mA rms.  I don't know the frequency, but it is frequency dependent.    I believe 60Hz is right around the best frequency for electrical damage, if not perception.  This will also be surface area dependent, and dependent on the types of receptors being stimulated, as well as their density.