Turlough's choice has the advantage of the fact the has had more screen time previously, we've seen him opting out of actually killing the Doctor several times and covering up for it by various sorts of weaselling. I'm amazed The King's Demons rated so highly, though its a while since I've see it. I would have thought both Terminus and Snakedance would have been strong contenders though (particularly the latter) depending upon your preferences in story telling.