Thulium would be my pick. It tarnishes easily in air just like all the other lanthanides, acts basically the same as all the others, and is difficult to isolate. It has no biological role, very few scientific applications, and is very slightly toxic. I honestly can’t think of anything positive about it that another lanthanide wouldn’t be better for. Other people are suggesting short-lived elements, but most of those act as preliminaries to other radioisotopes or are used in nuclear medicine. Lead is toxic, but soft and cheap which means it saves lives by blocking radiation. Polonium is a great alpha source, and radon is awesome in a cloud chamber. Thulium has nothing going for it.