It depends on your type of online course and your end goal. If you are planning to sell multiple online courses then Udemy is a perfect place to get a quick exposer and affiliate marketer to promote your course. You don’t have to do anything or pay any extra for the Udemy affiliate program.
Second is if you plan to sell a single course program or membership then choose Affiliate marketing marketplaces like ClickBank or JvZoo. On Clickbank, you have to pay 49.95$ one-time fee to list your products in the other hand JVZoo is free.
Another alternative is Zaxxa for Information based product launch and affiliate marketing tools.
If you plan to sell multiple course programs with the up-sell and down-sell methods then ClickFunnels is the best choice. It will cost you little extra money then other option but if do research on the cost of running own information-based business then it is nothing.
I hope it helps you.