The industry with the steepest markup (greatest profit margin) is cosmetics.
I can't quote you chapter and verse but I've heard several respected pundits say that.
Being most profitable and having the largest profit margin can and usually is a totally different thing.
The oil industry has a great deal of money invested in refineries, pipelines, etc., but they also make a lot of money (i.e. sell a lot of product), but their profit margin is not the highest.
Making a $100 million profit on $1 billion investment is only a 10% profit margin, whereas making $10 million on a $20 million investment is a 50% profit margin.