There's a mission given to Franklin by Lester (I think it's the first one from him activated by a payphone) that mentions it will make Debonair Cigarettes stock spike. I had about 35k on Franklin when it started, so it's slightly later on. It's a time limit mission, so when you get it, kill yourself and don't retry, get all characters to dump their cash into the company, complete the mission and then sell. Made so much money with that it was insane.