The truth, in a way that isn't overly critical of my current employer and emphasizes what I hope to get from a new opportunity. I take it less as "what's wrong with the company you're leaving" and more "what are you looking for from a company you're interested in jumping to", which has very similar answers but focuses on the positive and also gives them actually relevant information that they can use to help you. They can connect the dots that the reason you're looking for that is because you're not finding it there, without you having to trash a company yourself.