I believe this depends on what YOU like: do not do anything you don't LOVE to do.
Based on the options you provided, I believe the best skills anyone can acquire are in the marketing and sales fields.
These are the two fields that will allow you to do anything you desire because when you bring cash into your company or someone else's with those skills they will NOT let you go—and you as a business owner will continue to do WELL!
Consulting is what I do and where my focus is these days. Because of my sales and marketing background, I provide a better service to those I help by combining insights about my expertise, eCommerce, PLUS how to turn that into sales for their business.
In other words, anyone can build a site but not everyone can make it profitable—that's where the sales and marketing experience comes in!
Let me know if there is anything else I can help you with.
