Tres Bien.
Heroin tops this list.
While I'm sure no one needs reminded why the H is bad news, my naturally addictive and binging personality only exacerbates this. Still... someday, perhaps.
Crack is kind of the same way, only cause I've heard so much about it. It doesn't have heroin chic, though; in fact, quite the opposite.
Opium, due to it's classic associations.
That's about all. I don't think I ever really performed proper experimentation with hallucinogens... no third eye for me. But eh, what're you gonna do.