Well, I would like to try almost all drugs listed in Bluelight Psychedelic Drug Index 
But, speaking of "rare" substances, I would like to try:
1) Mescaline or LSD - not sure which one I want more
2) 2C-B / 2C-T-7. Not quite rare, but illegal and hard to get for me. Of course, there are online vendors, so I will try these two compounds sooner or later.
3) DOC or DOM
4) Proscaline - shouldn't be very psychedelic, but I still would like to try it.
5) DiPT - don't look rare, and I am going to try it within next few months.

But, speaking of "rare" substances, I would like to try:
1) Mescaline or LSD - not sure which one I want more
2) 2C-B / 2C-T-7. Not quite rare, but illegal and hard to get for me. Of course, there are online vendors, so I will try these two compounds sooner or later.
3) DOC or DOM
4) Proscaline - shouldn't be very psychedelic, but I still would like to try it.
5) DiPT - don't look rare, and I am going to try it within next few months.