The thing that makes meth heads and crack heads dangerous is primarily that the users are desperate for money to feed their addiction and thus more likely to commit crimes to make money. Coke heads probably started out fairly wealthy, since it's a more expensive drug, so they are less likely to resort to theft to pay for their addiction. It's probably also somewhat of a social class issue. Crack and meth users are probably more likely to have grown up in rough circumstances that make them more likely to commit violence whether or not they are under the influence of drugs. Reporting bias is probably also a bit of an issue. If a rich coke user assaults someone they may be able to sweep it under the rug by writing a big check to the victim, whereas a poor user would be thrown into the criminal justice system. A final hypothesis is that cocaine users have more resources for dealing with addiction (such as rehab) than other drug users. That would skew the population of active coke users towards recreational users rather than addicts.

There are chemical differences between crack, meth, and cocaine, so it's likely that, ceteris paribus, crack users are more violent than coke users, but it's not the only factor. It's really hard to tease out differences in the effects of the drugs when the populations of users are so different.