I guess nicotine is the only thing I've really been addicted to long term, so I could say nicotine. Maybe I'm addicted to weed I smoke everyday and it is far more important for me to get stoned then have a cigarette, but I've never wanted to stop smoking weed. Ketamine is the only thing I've been addicted to that felt like it was getting serious and out of control although that was a fairly short amount of time compared to how long I've smoked cigarettes or weed. I'm inclined to say heroin is the most addictive drug I've done although I've never gotten hooked on it. I think it would be hard for me to get addicted to any drug that's not ketamine because after using any other drug I've only ever found myself craving ketamine afterwards, not more of said drug (including when I've done heroin, meth and coke).. pretty much if i can get ketamine that's the drug I'll be doing.