Psychologically, cocaine. Probably over 80% pure, 30 grams in as many days. Crack, too, not flake. The withdrawal lasted about a week or so; the first three days were mostly dominated by immense cravings for cocaine and my thoughts constantly returning to it, while feeling paranoid, restless, anxious, and depressed. The rest of the week the cravings subsided as the other symptoms grew stronger, as well as the classic, almost cliché but very much real audio/tactile hallucination of insects crawling on my skin. For that week I took threshold doses of amphetamine sulphate to help with cocaine cravings, and ate levomepromazine/methotrimeprazine every night for insomnia and psychotic symptoms.
Nowadays I'm wary of using cocaine with any kind of regularity, and can use it in moderation, and not too often.
Physically, lorazepam. Even with therapeutic doses and after only 2 months of use, the withdrawal symptoms were completely unbearable, much worse than with any other benzo, even after longer periods of use and higher doses.
I've tapered off that now, and feel good, as I've also overcome my acute problems causing panic disorder, and learned to deal with it better.
Cannabis is my most persistent mental addiction; I've not been able to keep to a promise of "I won't smoke today" a single time, and last Christmas was the first time I'd spent a day without weed for over 4 years, I think. I don't see myself kicking it either, although I will have to start smoking less at some point.