It’s a toss up for me between a beaded chiffon evening gown, and a tulle ruffled ballgown. The beaded chiffon was the most challenging in every respect- keeping the silk chiffon from distorting and the tension correct as is a normal challenge, but with the additional difficulty of having to clean the beads out of the seam allowance and tie off all the strings so the beads wouldn’t unravel from the interior of the dress, and then using a zipper foot to construct the whole thing and not hit the beads with the needle! The tulle dress was 3″ ruffles on a circle skirt, which wanted to hang out on all the bias. Keeping the ruffles running in perfect horizontal lines while also dealing with the razor like cut tulle, and again keeping the machine perfectly tensioned was all challenging. I also burned out one machine using my ruffle foot for days and had to send it in for repairs! Great question.