Meth and coke/crack. Stims in general are really addictive, even the shitty ones like methylphenidate or amphetamine (amphetamine isn't shitty IMO, but it has a few side effects which can be bothersome to most, especially those who are not particularly tolerant of depression and anxiety) are fiendy as hell, making you redose compulsively (more true for methylphenidate than for amphetamine) until you either run out, or realize that you havent gotten high once during your entire binge, but you're in peripheral hell.
Also i'd be an idiot not to mention nicotine and caffeine, the two most used psychoactives in the world as far as i know. i find that coffee in particular is more addictive than other caffeinated drinks, i think there may be some other chemicals we havent found yet that are in coffee, like some other xanthines. nicotine is easy to stop, but once i get going again (usually triggered by amphetamine use, which goes with nicotine like jam and bread), i smoke a lot, and quite regularly, ranging from a 1-1.5 packs daily, sometimes hitting 3 packs in a night if i am drinking or using a stimulant. however, when i stop temporarily, due to lack of funds or health concerns, i find it pretty easy. i get the occasional craving, but nothing too intense at all. i can also cmoke on 1 night and not smoke for a while after. i only start using a lot once i buy a pack.