There is no point in talking about "what's the best online business". It's best what you feel that you can do best, mate.
I agree with Clive, what is your experience ? then create an online business based on your knowledge.
Its depend on your capability. Its different people from people. For me i think that by dropshipping is the profitable biz for me.
Google adsense is best business in the world...... and google now rejecting many of them and even not selecting new website becasue there is do's and don'ts in google adsense ,try the innovate new think and to get select in google adsense http://boomint.net/2009/05/07/google-adsense-dos-and-donts/ all the best
I think the simplest and one of the best businesses on the internet is freelance business. You can earn some really good money at freelance sites via doing simple jobs like data entry, trying, writing etc. And most importantly, many freelance sites are free to join. So this online business is free to join.
Every business is good and profitable you just have to find what works for you. You can sell anything on the net as well as offline.. from services to whatever.
To start one easily, you can stick with Adsense ads with your websites. But the crucial point to this is traffic and CTR.
I always answer a question like this the same way. Start by looking at something you are good at. Can you write? - Make money writing. Can you design?-Make money designing If you know about dog breeding, write a book about it and sell that. There are many ways its up to you.