Nicotine, one of the most addictive substances you will ever know. Nicotine is the reason you continued to smoke after trying it a few times; nicotine is the reason why you have had trouble quitting in the past; and nicotine is the reason why you feel so poorly during the first days or weeks after you have quit.
I would have to say that they all are in one way or another. Don't be fooled by cigarettes that claim they are all natural.