The single coolest thing about working in the animation business is that every project is jam-packed with its own set of challenges to overcome. Every sequence in that project has its own group of characters and performances and obstacles. And each shot within that sequence is overflowing with its own dizzying array of decisions, choices, and difficulties.
As such, this job never gets boring. Not even a little bit! You're constantly learning about new things that apply to one scene or another - be it dance, kung-fu, acting, car-racing, or how airplanes work. This is a career where you are basically signing up to be a student of EVERYTHING for the rest of your life, so buckle up for a nonstop adventure of new ideas and topics to research!
That said, if I had to choose one project as the most challenging, I think I'd have to pick Transformers 2: Revenge of the Fallen. I worked a lot on a character called Devastator, and figuring out how he was going to form together, rise up, and head off to rip down the pyramids was easily the most complex thing I've had the chance to work on, but also one of the most fun things I've ever animated!
Luckily, ILM has some absolutely incredible creature rigging geniuses (a word I don't use lightly) such as Kaori Ogino and Keiji Yamaguchi. Kaori rigged the creature, probably the most complicated rig ever done in CG, and Keiji was instrumental in helping animate some of the actual transforming geometry of the tractors coming together to form our big baddie.
Of course there are a lot of other people who come together to create something like Devastator, it certainly wasn't just the three of us, but I really felt the pressure on that character, both internal and external (I wanted it to be as cool as possible, just like everyone else did!) and as such, I have to say it was the biggest challenge I've faced.
But how cool is animation as a career when the biggest challenge is also the single most fun experience you've had in that career?
Man... sometimes I really love this job.
Thanks for the fun question!