There are thousands of different drugs so I'll just have to list a few:
Very harmful: Heroin, Crack cocaine, Alcohol, Tobacco, Crystal Meth
Quite harmful: Amphetamines, Benzodiazepines (Valium, Lorazepam, Temazepam, etc), Cocaine
In the middle: Ketamine, PCP
Not too bad: Ecstasy/MDMA, Amyl Nitrate, 2CB, LSD
Fairly harmless: Cannabis
This list that I've made is a combination of potential physical, psychological and social harms, not just physical harm. If it was only physical harm then LSD and Ecstasy/MDMA would be in the 'fairly harmless' category with cannabis, and heroin/crack would be in the 'quite harmful' category, because they're nowhere near as physically damaging as alcohol and tobacco.