Business Insider says go short arms makers, pharmaceuticals, and alcohol and buy cigarette companies and purveyors of munchies.
Seriously though, legal marijuana will be seen as a direct challenge to the authority of the federal government who will certainly feel the need to assert their power. What will they do? Send the Marines into Humboldt County? I wouldn’t rule it out. This would almost certainly lead to all manner of social unrest and protest. States Rights has up until now been a right wing issue. With legal marijuana, it will almost certainly become a left wing issue too.
Then there’s the dirty banks and corrupt hedge funds who want drugs illegal because they make so much money from it. Because they don’t care how many more die in Mexico or who gets addicted as long as the literal blood money keep flowing their way.