It's a toughie, but one you can be sure you'll be asked. Definitely keep things positive-you have nothing to gain by being negative about your past employers. Instead, frame things in a way that shows that you're eager to take on new opportunities and that the role you're interviewing for is a better fit for you than your current or last position.

For example, I'd really love to be part of product development from beginning to end, and I know I'd have that opportunity here." And if you were let go? Keep it simple: Unfortunately, I was let go, is a totally OK answer.