Never badmouth your previous industry, company, board, boss, staff, employees or customers. This rule is inviolable: never be negative. Any mud you hurl will only soil your suit.
Especially avoid words like "personality clash", "didn't get along", or others which cast a shadow on your competence, integrity, or temperament.
BEST ANSWER:
(If you have a job presently)
If you're not yet 100% committed to leaving your present post, don't be afraid to say so. Since you have a job, you are in a stronger position than someone who does not. But don't be coy either. State honestly what you'd be hoping to find in a new spot. Of course, as stated often before, you answer will all the stronger if you have already uncovered what this position is all about and you match your desires to it.
(If you do not presently have a job.)
Never lie about having been fired. It's unethical - and too easily checked. But do try to deflect the reason from you personally. If your firing was the result of a takeover, merger, division wide layoff, etc., so much the better.
But you should also do something totally unnatural that will demonstrate consummate professionalism. Even if it hurts , describe your own firing - candidly, succinctly and without a trace of bitterness - from the company's point-of-view, indicating that you could understand why it happened and you might have made the same decision yourself.
Your stature will rise immensely and, most important of all, you will show you are healed from the wounds inflicted by the firing. You will enhance your image as first-class management material and stand head and shoulders above the legions of firing victims who, at the slightest provocation, zip open their shirts to expose their battle scars and decry the unfairness of it all.
For all prior positions:
Make sure you've prepared a brief reason for leaving. Best reasons: more money, opportunity, responsibility or growth.