Sadly, there's no one-size-fits-all answer to this question. The industry you choose, the product you sell and the niche you approach are all dependent on you, your personality, your experience and your network.
So this is what I wrote back.
Hi, Phil (not his real name). Thanks for your note.
I would not presume to advise you without knowing a little more about you. What work experience do you have? What are you best at? What do you most enjoy doing? Do you enjoy working with people? How much money do you have to invest in a business?
These are all crucial questions that can help you or others hone in on what business is right for you.
I'll let you know if I hear from him again.