Easily the strongest and most powerful addiction in my memory would be the videogame League of Legends. I'm talking playing all night, anti-social behavior (in freshman year of college, this is practically impossible), and a reckless disregard for my responsibilities in life. I would push off essays and research reports, just naturally assuming that I would stay up all night to finish them, and instead play League till 1 or 2 am, early on, and till 5 or 6 am, later in the year. Of course my addiction to the game was abetted by an absurd caffeine dependency and by my prescribed Focalin (though I was never even remotely addicted to this compound itself--I actually didn't even use it as often as prescribed, which is what allowed me the "extra supply" to pull all nighters with).
The end result was messy. I first noticed psychosomatic symptoms--nervous ticks like throatclearing, which eventually progressed into a full-blown paranoia around swallowing. I believed I had something stuck in my throat, you see. It became incredibly difficult for me to eat, for the simple reason that I had "forgotten" how to properly swallow, and instead tended to choke on or partially inhale food on a daily basis. I couldn't sleep when I tried, and I couldn't focus when I tried. I existed in an ugly state of "comedown" perpetually--anxious, unfocused, sleep deprived, irritable (relative to me, I'm very easy-going). I got behind on work, and was put on probation after the semester's end.
I was asked to leave the semester after that, but that's another story--though I played some League during that semester, my addiction to it was mostly broken--supplanted, I think, by my "addiction" to a girl who came into my life. And, while that had problems of its own, in terms of potential for distraction, I can confidently say that it's a much more psychologically healthy addiction to have. In any case, after contracting mono this next semester I was asked to leave, which was of course very difficult for me.
Since then I've replaced my focalin script with Vyvanse. While no medication is without its faults, the Vyvanse has been a lifesaver for me in terms of it's long-lasting, powerful, and clean stimulation. I no longer suffer the abrupt ups and downs of Focalin, or the horrendous comedowns, or the painfully intense appetite suppression, or the "cracked out" antisocial alien feeling. I can focus in class every day, and even when my procrastination catches up with me, a vyvanse-fueled allnighter leaves me in a far more functional, "normal" state the next day than Focalin could ever provide--which actually scares me, to a point. While I have seen the "dark side" of focalin because it's readily apparent, I think the dark side of vyvanse is how incredibly innocuous it feels. This is why I don't abuse stimulants for the high.
Anyways, sorry about the rant. But for reference, I've got experience with alcohol, cigarettes, weed, RC psychedelics, mushrooms, kratom, various opiates, phenibut, yohimbine, etizolam, and an assortment of other nootropics, RCs, etc. Nothing has ever made me want more like League--with the possible exception of psychedelics, though they're self-limited by tolerance. I don't know what that says about me. I've always been an avid reader of fantasy and sci fi literature, maybe I just like to escape.