LSD is the most powerful psychedelic drug on the planet.
Salvia divinorum (or more accurately, its active principal salvinorin A) is the most potent psychedelic compound known, at least in its pure form anyway.
Late Correction: actually I'm wrong. By weight, salvinorin A is still the most potent naturally occurring psychedelic compound, but LSD (semi-synthetic) still edges it out by a few micrograms.
That'll teach me to not hit the books before posting this shit.