It is difficult to compare issurim to determine which is worse. I think enough commenters here recognize the issurim on excessive alcohol, tobacco, and weed. Whenever I discuss any of these topics, I find those who wish to retain their freedom to ingest the substance as the most avid matirim, with some pieces of logic that any toddler would reject. So let’s recap a bit. Cigarettes are lethal to the smoker (and pretty bad for the second hand smoker, too). Smoke does not belong in the lung. Only clean air does. All the dangers of tobacco smoke exist with cannabis smoke, plus lots of others. Tobacco creates physical dependence. Marijuana does not. Actually, NIDA has published evidence of physical dependency to marijuana, but this is definitely not the most common or dominant issue with it. Alcohol affects many organs, including liver, pancreas, stomach, brain, and more. There is brain damage from excessive alcohol. Nothing near that is found with weed.
The medical damage from weed is more insidious. It appears with little to no drama. Rarely does MJ send someone to the emergency room. It becomes hallucinogenic in very high doses. This can occur with anyone, but it is rare that someone bothers to ingest that much. But the damage is more subtle, and one does not see it happening at all, so there is no motivation to abstain. That makes it more of a danger.
MJ – Long term damage to lungs, glands, and brain. Short term memory and reaction time impairment. Makes one stupid.
Alcohol – Intoxication and poisoning, physical dependency, damage to gut, brain, liver, and pancreas, among other organs.
Tobacco – Heart, lungs, and cancer.
These are just a sampling of the headlines of dangers associated with these 3 substances. There are also a variety of issurim. My Torah tells me that every לא תעשה is bad.