Cocaine... ugh. It sometimes seems to attract a bad "type" of people, and turns people into egocentric dickheads, sometimes violent. Plus, it's cut with crap and it's expensive too. Doesn't last long either... and then it's just a night of constant fiending. I'm not really experienced with coke though.
Alcohol has a very ugly side too. Can be quite a destructive drug, although I think in moderation it's fine for most people. There are so many things I could say about alcohol too, but I might as well just leave it here.
I'm not experienced with opiates but they seem like they could be pretty crazily destructive too, although I'm sure they have some positives.
Now MDPV's an odd one. It's not as egotistical as coke, but it is fiendish as fuck. At least it lasts longer, and at the time, was very very cheap. But the comedowns can be fairly hellish. It's not just the comedowns but the restless nights... But MDPV taught me that dopamine's a bitch, but it's for your own good. You're a slave to a chemical, a drug of your body's own design.
I hate methylone somewhat too because it my use of it fucked a few things up for me... But I would pretty much always snort it. And again, the comedowns, ewww. I hate the manic, grandiose element of the high, plenty of things to regret here. I think taken orally or very small doses it's quite a nice drug though.
Tobacco, to an extent. I hate how my addiction's just got me by the balls. It makes me feel weak, because I don't seem to be able to stop. I know I want to quit someday, but I can't bring myself to actually do it. Just another cigarette. I like it a lot for a few reasons, but it's made me realise how strong I should be, how I'm nowhere near fulfilling my potential - yet. Aside from addiction, things like cancer, lung and mouth diseases are really nasty too. I once went to visit my great aunt before she died and she had this hole in her neck, I think that it had had to be cut out because of a tumour or some kind of smoking related growth or problem... Just thinking about that really fucks with my mind. It was horrible.