As far as I can remember from pharmacology crack cocaine is a drug that is 100% addictive in that you get physical withdrawal symptoms after the first time you take it. However Roisin probably knows a lot more about it!
There are different conclusions in various research papers, but the same names come up time and time again, with just a small variation in how they’re arranged: according to most research the most addictive substances are nicotine, smoked methamphetamine, heroin and crack cocaine. Some rank heroin at the top, some rank nicotine at the top, but those four are nearly always the top four.
In terms of ‘capture rate’ that is, the percentage of users who reported that they became dependent on their drug of choice, the order is tobacco/nicotine, followed by heroin and then cocaine, so you can see that the same type of trend is maintained.