As someone who's been using meth for 14 years, mostly IV, I feel the need to point out that you can't judge all by the most visible examples of a particular drug's users, regardless of what that drug is.
I'm a little off, but that's genetic, not environmental.

I have all my teeth, and rather nice they are. I'm strong, heathy, and at 130, certainly not starving.Don't mind my modesty, but I'm a young-looking, attractive woman at 31(did I really just type that?Ha.) Before moving to MS, I had a $26 dollar an hour job, with full benefits (after dropping out of school to pursue a short but brilliant career as a heroin junkie).
I choose to live a fringe lifestyle, but that suits my personality,and my former street-kid mentality. I am not an obvious drug user, visually. I think my tattoos are more harmful to my future than drugs.lol
Yes, the ones that 'fall', fall hard, but like any other drug, for every user who is an obvious mess, there are 3 in the shadows, living normal but secret lives.
In answer to the actual question, I'd have to say MDMA, because of its powerful effect on the brain and psyche, but mostly for the wild unpredictability of never knowing what you're actually getting.