Well, it depends. If you're looking for a car rental discount with National Car Rental, Disney Club will offer the best deal. (Amex does not offer such a discount.)
For a discount on Park Hopper passes or room rates, Disney Club and
AAA are about identical - about 5% off passes and up to 20% off rooms. (But the best discounts on rooms are often code discounts, which are available to anyone, and Annual Passholder discounts.) Amex won't help you with passes or rooms.
Once you're at WDW, many of the Amex discounts are better than Disney Club. For instance, you can get 10-20% off lunch at most of the restaurants in Downtown Disney with Amex, but only 10% off at Rainforest Cafe (with a coupon) from Disney Club. It's like that pretty much down the line -- Amex gives you discounts on tours, boat rentals, dinner shows, DisneyQuest admission, etc. DC doesn't. Once in awhile DC will have an on-site discount that Amex doesn't offer (like right now you can get 30% off golf fees), but not often.
I have a lot of the various on-site WDW discounts listed here so you can compare them:
http://www.mousesavers.com/entertainment.html
I really don't know what DVC will offer you, sorry.
Mary
MouseSavers.com