See: Merrill, D. R., Bikson, M., & Jefferys, J. G. (2005). Electrical stimulation of excitable tissue: design of efficacious and safe protocols. Journal of neuroscience methods, 141(2), 171-198.
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See table on page 185 but note that this is based on experiments where the material was implanted in nervous tissue and not used as an electrode. When you pass electricity through a metal especially it will tend to ionise and form metal ions in solution. This could be dangerous in the case of stainless steel (listed in the table as non-toxic, when not used as an electrode) as the chromium in it could form highly toxic and cancer-causing Chromium VI ions at certain electrode voltages. Stainless steel also contains Nickel which is also toxic. Obviously anything chrome-plated should be avoided.
The table also lists aluminum as non-toxic but there are studies implicating it in Alzheimer's-like brain damage. When you use it as an electrode it will release ions.
Carbon (eg. conductive graphene condoms, graphite in conductive rubber) is safe.
Gel electrode pads are safe.
The safest metals as electrodes are gold and platinum.