In my opinion, Kiwitaxi has an excellent affiliate program with informative statistics and a handy set of tools for developers. Amateur webmasters can use ready-made forms of search, and more advanced ones can create their own form based on the existing API.
For a couple of years of being an affiliate, I occasionally had to contact (via VK and Skype) support for help and always received detailed answers regarding programming, financial issues, and even tourist destinations popular at the moment. In additional, payouts are always accurate!