there are more suited places to talk about things like this, but since the OP was talking about MDMA and mentions "countless substances" - this is quite a nice

there are more suited places to talk about things like this, but since the OP was talking about MDMA and mentions "countless substances" - this is quite a nice one https://www.google.de/#q=shulgins+favourite+drug https://www.google.de/#q=shulgins+favourite+drug although you could argue that synthesising and experimenting with unknown substances might have been his real favourite thing