Hard to say, I'd have to pinpoint a few drugs for different reasons.
As funny as it may sound, the drug that I have had the hardest time not using is cannabis. Ever since I started toking daily, if it's been available there haven't been many days on which I haven't smoked it. For the last 2-3 years, there's been less than 10 days when I haven't smoked weed - and I've tried to make commitments not to smoke, only to fall to temptation in the evening. I guess it's the fact that I like it so much added to its relatively mild side effects - I can't and don't feel that guilty for smoking weed, since I don't perceive it as that harmful and I still get my shit done, more or less.
Benzos have been the nastiest addiction to kick, by far - it's taken shitloads of willpower and I've gone back and gotten myself addicted again several times. Also I've been so dependant that not taking them made me completely dysfunctional, a paranoid agoraphobe.
Cocaine has struck me as a particularly addictive drug, in that I don't subjectively enjoy its effects all that much - nevertheless I have always finished whatever cocaine I had in just about one sitting, as it is incredibly fiendish. Also, after I stop using coke, the next day I feel completely baffled: I have an inexplicable craving for the stuff, even though I'm constantly telling myself "But I don't even like the stuff, I didn't have fun last night."