The problem with vice-type industries like gambling, cigarettes, and junk food is that one can make a reasonable argument that the users are deriving some kind of utility from gambling, smoking, and stuffing their faces with Cheetos. How do you measure utility? If it's strictly in terms of WTP (willingness-to-pay), then vice doesn't seem to be such a good candidate, you need to look at industries where there is an element of force or fraud, but not so extreme that the industry is banned.
Also one needs to distinguish between utility at an individual level and utility at a societal level. In arms race type situations, things which have a lot of utility at an individual level have very little utility when you look at things from the perspective of the entire universe of consumers.
That said, my vote is for higher education. At an individual level, lots of bright people go deep into debt and waste the best years of their lives because they are led to believe they will get good jobs which never materialize. At a societal level, a large portion of higher education is basically arms-racing, also known as credential inflation.
One could object that most higher education is performed by non-profitmaking institutions, but I think that is a legalistic limit on the definition of "profit" which is not true to the spirit of the question. Universities and professional schools generally do very well and pay their senior management very handsomely.
This is an offshoot of a thread I made earlier, but which wasn't eliciting the sort of responses I'd hoped for.
So let me pose a clearer question with less potential to get people on watchlists.
What legal ways of making a profit are the most anti-altruistic, the most damaging to society, the opposite of effective altruism in result.
I am using utility loosely. The answers need not be given from a utilitarian perspective at all, but instead merely deal with any means of making a profit that seems to you clearly wretched, and such that the world would be better if nobody participated in it.
I'd also like to emphasize that these things should be legal. There are some obviously wretched illegal businesses that would top the list otherwise. If something is legal but only in a particular jurisdiction, then you should only discuss it within the context of the jurisdiction where it is legal.
If it's a grey area, go for it, but extra points for society-harming enterprises definitely legal in both the letter an the spirit of the law.
This is NOT about whether the enterprise in question should be illegal, just whether it causes a net loss of utility (deal with counterfactuals however you see fit).