Well everyone has their own reasons for voting a particular drug the worst. But the ones I personally am going to stay away from are coke, tamazapam and hard liquor.
My first time snorting a line I was instantly wanting more ... ten times worse than going on an alcohol binge. And that super-cool, detached feeling of: 'I'm the shit, I'm perfect' is an unhealthy state. The aftereffects of the high are just so edgy too ... at least for me ... a twitchy nervousness that immediately calls for another line.
For my second worst drug: twice I tried to get high on tamazapam and that's two nights I have ABSOLUTELY no memory of. Each time I took at first 3 pills hoping to catch a buzz and drink a little for escapist bliss. Both times I woke up having no recollection of what happened and finding that I had taken ALL the pills. apparently this stuff is hard to OD on or else I'm just very lucky.
Any drug that awakens a craving for more and more and more freaks me out. I also experience this with alcohol but now I know my limits and am able to cut myself off after four drinks, knowing that more will make something in my head snap and I'll be unsatiable. And I only drink beer ... no whiskey!