Having only conducted at lower section level (phew! ;-)), I won't have come across the more recent test pieces at top level, but this one's for all conductors whether you've conducted the cream of the crop or not. So what's the piece (doesn't have to be a test piece) that's given you the most nightmares regarding beating time, interpretation etc. Funnily enough, what I consider the most difficult piece I've had to conduct occurred in my contest conducting 'debut' in 1998! Though there are pieces like Bourgeois' Serenade that are initially hard to get to grips with, the phrasing actually helps after a few goes (for me). Nope, the most tricky piece I've conducted is what I'm currently listening to on my headphones.... An Epic Symphony! One of the most difficult works I've worked on, certainly for indicating where the next beat's coming from in the slow tempos (in both the first and second movement) and shaping the phrases therein whilst trying to indicate the beats adequately. Still gives me the willies thinking about it! What's your conducting Nemesis?