The walk cycle. I found this extraordinarily difficult, just the logistics of what moves where and when combined with the timing of each mmovement. Also applying the vague principles of human anatomy to the character I designed (I called him mittenman because his fingerless hands resemble mittens). So while its only 19 frames it took me a good 3 hours to fantastically inefficient, new found awe for traditional hand drawn animation all round. 3D next!