The one you are passionate about and will gladly give every available hour to for the rest of your life.
There are always options but if you love what you do your business may not be limited to just one option. My business is for example an extension for my passionate love for online and social media marketing and blogging. I monetize them with 1 Network Marketing Program, Pay Per Click earnings, Direct sales advertising, and affiliate marketing.
My family and I make jewelry and I love buying and selling collectibles so our other business is a e-commerce business and we augment earnings for it by promoting our various sub stores like TripleClicks and Amazon through their respective affiliate programs.