I have found most UX projects to be exciting and interesting because I absolutely love what I do. That being said, the most challenging (and therefore rewarding) would have to be designing for accessibility.
I was responsible for the redesign of a banking CRM application to support partially sighted or blind people to be able to work effectively in a Contact Centre. This project was more about HCI and usability than it was visual design but it provided invaluable experience in research, design, usability engineering and WCAG which is totally relevant to website design and largely missing in the profiles of many UX professionals today.
I worked along side an inspirational man who had almost completely lost his sight through cataracts and was assisted by a student PhD psychologist and a couple of very talented developers. We worked with text equivalents so that the screen reader software would work effectively and the CRM application and softphone interfaced with a brail keyboard controller.