Different types of content with different types and size of audience will demand different types of revenue models. So a free model will not work for everything, an ad model will not work for everything, and a pay model will not work with everything (at least on the internet).
I'd say freemium is the most promising approach so far where those who are willing to pay more pay a lot more to cover the costs incurred by those who aren't willing to pay anything at all, so that those aren't willing to pay anything at all make the service/product more valuable to those who pay more. Whew... Anyways airlines operate sort of like this where First class pays a shitload more than coach. The graduated tax system is another example.