Hector relax is one of the more rare things, then there are things that have only been given out a couple times. One of the most prized things any flame collector wants is flame ID 1, made by hampa. Currently Toast owns it and has no intention of selling it.
Full 512 joints used to be up there when only 3 people had them, but now a lot of people do.
There's only one item that I can think of that has only 1 in existence, and that's the NeckBeard. Currently owned by Risk, it was a christmas gift from Solax. I think Rekt had/has one too, which was also a gift from Solax.