To talk about a specific client or project that I consider to be the most challenging is quite a difficult task, not because there are too many to choose from, but because every project I've worked on has taught me a unique lesson and has helped me to get to this position, even if at the time it might have caused many sleepless nights!
The projects I've found the most challenging have largely been due to language barriers. At Green Chameleon we really do enjoy working with clients in all different parts of the world however it has been a big learning curve understanding the requirements of clients that don't speak the same native language and come from completely different cultures. However, once this barrier has been overcome it's always reassuring to see that good design is something we can all agree on!
I can think of one occasion where we received a lot of criticism. It was the launch of our first agency website, the site was not fully responsive and this was long before responsive sites were delivered as 'standard'. At the time we were just happy to finally get our website out there however it wasn't long before we received quite a big backlash on Twitter with people moaning that a design agency shouldn't have a non-responsive website then claim they can build them. It was frustrating as we had spent a huge amount of time on the project and people didn't know the full story. I have to say it made us consider our approach and ensured that we would never get caught out again. We decided then that every site we produce needed to be responsive.
Overall, my favourite and most satisfying projects to work on have been the ones we've done for ourselves, outside of the usual client work. For example our own GC rebrand last year and the 'Candy is Dandy' food typography poster plus various speculative concepts I've designed just for fun. I think that this is largely due to the creative freedom we have on self-initiated projects that makes them so interesting to work on. I've also found that they have received the most compliments and in a lot of cases gained us the most attention. For example my debut shot on Dribbble was a speculative concept for Specialized bikes which has easily been my most complimented and appreciated piece of work to date. Other memorable points in my career have to be receiving our first feature in web designer magazine and of course I can still remember the joy of being awarded our first CSSDA WOTD!