Well I've smoked many weird substances, that's for sure.
But the nastiest I've ever smoked has got to be those fucking Yopo/Cebil seeds! AWWRRHHHGGG, DISGUSTING! That shit tasted like woody, burnt salmon! No joke!

In case you don't know what those are...

Anadenanthera colubrina
http://www.erowid.org/plants/anadena...nanthera.shtml

They're supposed to be used for a snuff.. but I was ovah that.